World Register of Marine Species is excellent for marine species. Text information is covered by a CC-BY-SA license and can be copied to Wikipedia (using the proper citation). It is mostly fairly accurate, and is being revised constantly.
Animalbase has lots of info on land/freshwater species, usually European. Authority references for all species and genera. The type species is marked in bold text. All data, all images and all texts at AnimalBase are public domain. (Comment: all AnimalBase content was considered to be public domain since its start, but since July 2010 its public domain status is clearly mentioned on its homepage.)
Note: not all the new taxonomic changes are adopted in AnimalBase, for example see [1].
Paleobiology Database is excellent for families, lists of genera etc, not only for fossil taxa but for some living taxa also. -> Fossilworks. Note, that Fossilworks does not show parenthesis for species.
For genera only. Use it to look up authority and date for a genus, and to confirm the spelling of the genus. Very good indeed, but apparently not quite 100% complete. However, the authority references for virtually all gastropod genera can be obtained from this source.
By William B. Rudman. Very good indeed, but remember this site includes cutting edge research, so sometimes it includes as yet unpublished opinions on taxonomic questions. Unfortunately no longer available.
A blog by Aydin Örstan of the Carnegie Museum. His blog entries do sometimes show up in google searches, and sometimes the info is useful to us here on Wikipedia.
Taxonomic Database on European Marine Mollusca, maintained by Muséum National d'Histoire Naturelle,
Muséum National d'Histoire Naturelle (Department of Systematics & Evolution), Paris. This site has taxonomic information about all European marine species, including their synonyms, and authority references.
Contains a listing of many European marine gastropods. Daily updated by a board of taxonomic editors who are world experts on the taxonomy of their relevant taxa.
A site created by the Belgian malacologist (and shell dealer) Guido Poppe. Some of the taxonomy is quite out of date, plus some of the identifications are incorrect.
Just discovered this fantastic website. Provides links to several interesting tools, such as genetic and ontogenetic databanks, regarding several taxa.
A nice site for amateurs to send in their nudibranch pictures. Many photos are verified correctly identified by experts in the field. A good site to add as an external link to on a species page or to check what a species looks like for general reference.
ITIS was formed in 1996 and many of its records are not up to date. The site reports an update, but it contains extremely few records for land/freshwater gastropods, (and for marine taxa we use WoRMS). It can be uses for vernacular English names sometimes.
Note: Be careful with using taxonomic info from the websites of many shell dealers. Individual shell collectors' sites also range from good to unreliable.
Please help expand and improve these tables. Some of the sites are better organized and easier to search than others. Some are ones from which info shows up in google searches, but are not necessarily suitable to search routinely.
No one resource is complete for all information. In this Wikipedia project we are trying to use the most reliable taxonomic information. Currently (29/5/2010) that is true at least in the family articles, see also the resources listed in those articles.
Sites not yet assessed
Can I ask what other people think of:
http://www.bagniliggia.it/WMSD/WMSDhome.htm - may be useful to get at least minimal approximate number of species in a certain family (if the website uses up to date taxonomy)
