The taxonomy of many groups is in a state of flux as taxonomic experts strive to incorporate the findings of Molecular phylogenetics, so it is not always possible to find a single satisfactory classification, and we would be doing a great disservice by pretending otherwise. The best would be to try and find out what the current consensus is, if there is one, and make notes on variant systems. In this, the following resources may be helpful:

General taxonomy

• Catalogue of Life - The Catalogue of Life is the most comprehensive and authoritative global index of species currently available. It consists of a single integrated species checklist and taxonomic hierarchy. The Catalogue holds essential information on the names, relationships and distributions of over 1.7 million species. This figure continues to rise as information is compiled from diverse sources around the world. Results are explicit about their primary sources but may trail behind primary databases such as World Register of Marine Species.
• M. A. Ruggiero et al.: A Higher Level Classification of All Living Organisms PLOS One, 2015; also a few corrections here. A Classification of all life down to the level of order, to be implemented through the Catalogue of Life. Families are not included, although they can be found at this 2014 version here (as an excel spreadsheet, some orders etc. changed for the 2015 version)
• NCBI database − The Taxonomy Database is a curated classification and nomenclature for all of the organisms in the public sequence databases. This currently represents about 10% of the described species of life on the planet. It attempts to incorporate phylogenetic and taxonomic knowledge from a variety of sources.
• UC Berkeley: History of life through time − phylogenetic cladograms; many well-summarized groups with illustrations; many pages "under construction"; links to other useful sites
• Mikko's Phylogeny Archive - A private archive of various phylogenetic trees.
• Index to Organism Names - Clarivate Analytics (formerly Thomson Reuters, formerly Biosis) - zoological names only, taxonomic hierarchy is useful a starting point but not necessarily up-to-date or complete.
• The Interim Register of Marine and Nonmarine Genera (IRMNG) - A comprehensive source (at least to c.2012) for genus names and their authorities for "all life", taxonomy is fairly up-to-date in some portions (higher plants, algae, fungi), variable in others; not all genera yet allocated to families.
• Paleobiology Database [1]: taxonomic and distributional information about the fossil record of plants and animals, complete for many well-known taxa, less so for others.
• Systema naturae − usually gives multiple breakdowns for groups, which is sometimes confusing but can be very useful.
• www.itis.gov − now part of Catalogue of Life an automated reference database of scientific and common names for species built within a working hierarchy. ITIS partners with Species 2000 to build the Catalogue of Life. Covers a lot of ground, but is often incomplete or idiosyncratic. ITIS pages older than a few years or with no review date should not be used, and in general ITIS cannot be considered a reliable source on its own. (Note domain used to be www.itis.usda.gov . Deleting the usda component may restore the link.
• Species 2000 − now part of Catalogue of Life a list of specific taxonomic (current) databases, covering contemporary and fossil organisms.
• Systax - a database system for systematics and taxometry based at the University of Ulm, Germany, which can be used as an alternative to the Itis system listed above. ^{dead link}
• Kluge Principles of taxonomy
• Global Names Index - A repository of ~18 million names (species, genus + alternate names and attributions from author) from a multitude of source.

Marine organisms

• World Register of Marine Species [2] - an authoritative searchable database of marine organisms, from vertebrates to viruses. The taxonomic editors for each section are the experts in their group of organisms.

Animals

• Animal Diversity Web from the University of Michigan - Very informative
• Lepidoptera and other species (mostly related to Lepidoptera, such as popular butterfly plants, etc.). Info collected from other sources, not sure how accurate it all is. Interesting note: he has a (open source) perl script generating range maps automatically from distribution text.
• BugGuide.net--extensive resource on taxonomy and identification of North American arthropods. Experts in many fields visit and help with taxonomy.
• Diptera.info--extensive resource on taxonomy and identification of Diptera. Experts members help with taxonomy
• Hymis forum--extensive resource on taxonomy and identification of Hymenoptera. Experts members help with taxonomy
• Fauna Europaea - Database of all European land and freshwater animals.

Amphibians and Reptiles

• AMNH database - Preferred taxonomy used by WikiProject Amphibians and Reptiles.
• ITIS, Reptilia - Preferred taxonomy used by WikiProject Amphibians and Reptiles.

Birds

• Avibase - Database including all the world's bird species.
• IOC World Bird List - Preferred source of vernacular names and taxonomy used by WikiProject Birds

Brachiopods

• Emig C. C., Bitner M. A. & Álvarez F., 2019. Brachiopoda Database.

Fish

• FishBase - Huge database giving basic info on thousands of fish. Preferred taxonomy used by WikiProject Fishes.

Mammals

• Mammal Species of the World, 3rd edition (MSW3) - Database of mammalian taxonomy. Preferred taxonomy used by WikiProject Mammals.
• Mesozoic mammals - Containing much information about Mesozoic mammals and relatives.

Molluscs

• Gastropods (Mollusca). For all marine species, Wikiproject Gastropods uses the taxonomy curated by MolluscaBase and also viewed on WoRMS. This system is based on the 2017 "Revised Classification, Nomenclator and Typification of Gastropod and Monoplacophoran Families" by Philippe Bouchet & Jean-Pierre Rocroi, Bernhard Hausdorf, Andrzej Kaim, Yasunori Kano, Alexander Nützel, Pavel Parkhaev, Michael Schrödl and Ellen E. Strong in Malacologia, 2017, 61(1–2): 1–526.

• Bivalves (Mollusca). Wikiproject Bivalves is using the taxonomy of Bieler, R., Carter, J.G. & Coan, E.V. (2010) Classification of Bivalve families. Pp. 113-133, in: Bouchet, P. & Rocroi, J.P. (2010), "Nomenclator of Bivalve Families with a Classification of Bivalve Families" Malacologia 52(2): 1-184, which can be found at: Philippe Bouchet & Jean-Pierre Rocroi, Rüdiger Bieler, Joseph G. Carter, & Eugene V. Coan. 2010: Nomenclator of bivalve families with a classification of bivalve families. Malacologia 52: 4-112.

• Cephalopods (Mollusca). CephBase - Superb information about Cephalopod classification. Preferred taxonomy used by WikiProject Cephalopods. ^{dead link}

• Solenogastres (Mollusca). WP's taxonomy currently uses García-Álvarez, Óscar; v. Salvini-Plawen, Luitfried (2007). "Species and diagnosis of the Families and Genera of Solenogastres (Mollusca)" (PDF). Iberus. 25 (2): 73–143. Archived from the original (PDF) on 7 July 2011.

Platyhelminths (Turbellaria)

Baguñà, J.; Riutort, M. (2004). "Molecular phylogeny of the Platyhelminthes". Canadian Journal of Zoology. 82 (2): 168. doi: 10.1139/z03-214., reflected in "Turbellarian taxonomic database". Archived from the original on 2006-09-12. Retrieved 2011-02-12.{{ cite web}}: CS1 maint: bot: original URL status unknown ( link)

Arthropods

• Arachnida
  • Spiders - World Spider Catalog - Preferred taxonomy used by WikiProject Spiders
  • Scorpions - The Scorpion Files
  • Harvestmen - World Catalogue of Opiliones (The complete taxonomy of world harvestmen)
  • Other arachnids via World Arachnida Catalog: Amblypygi | Aranae (=WSC) | Palpigradi | Pseudoscorpiones | Ricinulei | Schizomida | Solifugae | Uropygi
• Centipedes - Chilobase 2.0
• Arthropods - Arthropoda Species File - includes Mantodea Species File (MSF) and other Species Files on various insect orders and other taxa.

Plants

• Angiosperm Phylogeny Group (2016). An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG IV. Botanical Journal of the Linnean Society, 181, 1-20. available here: Available online. The current authoritative source for flowering plants at family level and above, updated from APG I/II/III.
• Angiosperm Phylogeny Website : incorporates the latest taxonomic research; continuously updated.
• The Missouri Botanical Garden is searchable by species, and where this is available, will give the accepted classification, and the degree to which it is accepted. Also lists authors, synonyms and homonyms. Far from complete.
• Vascular Plant Families - a systematic and alphabetical index of the non-flowering and the flowering plant families with the Cronquist system and the phylogenetic system of Judd et al. (2002) (i.e. APG)
• USDA/NRCS PLANTS Database - Not complete, but nice. Resource for some PD images. Common names used are usually only common in the USA and may not be used worldwide.
• L. Watson and M.J. Dallwitz (1992 onwards). The families of flowering plants: descriptions, illustrations, identification, information retrieval. http://delta-intkey.com - Unique resource, updated to about 1998 (note that especially the lists of genera are outdated) - includes classifications of Cronquist; Dahlgren; Dahlgren, Clifford and Yeo; and APG (i.e. APG I, of 1998)
• System of Embryophytes Archived 2020-10-06 at the Wayback Machine by A. Novikoff & B. Barabasz-Krasny, 2015. A good modern treatment of fossil and extant plants (angiosperms excepted), particularly useful for fossil genera and families (with a small number of errors and some omissions). Not completely congruent with the treatment of modern plant divisions by Ruggiero et al, 2015 as given above; a version of Novikoff & Barabasz-Krasny's "System" with modifications for compliance with the latter treatment is included in IRMNG, the Interim Register of Marine and Nonmarine Genera, see listing above in "General Taxonomy".
• Gymnosperm Database - very good coverage of gymnosperms
• ILDIS A database of legume taxonomy, includes synonyms and accepted names, common names and detailed bibliographies for many species.

Likewise, the following sites can help find taxonomic authors and abbreviations:

• IPNI, authors search Note that the author database is separate from the plant name databases: the author database is authoritative. The plant name databases are "as is" and should be used as a search aid (invaluable as such) rather than as any kind of authority.

Fungi

• Index Fungorum - database of fungus species, genus and higher names, with all historical synonyms and indication of current name
• MycoBank - nomenclatural and taxonomical database, similar in purpose and coverage to Index Fungorum
• ITIS, which covers other kingdoms as well

Protists and prokaryotes

• BIOS database of bacteria, archea and cyanobacteria names, with bibliography.
• List of Bacterial names with Standing in Nomenclature - Bacteria have a formal approved nomenclature, all approved names can be found here.
• List of Approved Bacterial Names - list provided by the American Society for Microbiology.
• AlgaeBase - database of algae species, very large but not complete

Viruses

• International Committee on Taxonomy of Viruses (ICTV) [3] - the de facto standard for Wikipedia virus articles to method. A good place to see this in action and get an idea of how it works can be found at Virus classification.
• Medical Subject Headings (MeSH) database ( [4]) also is a useful resource for virus taxonomy.