Online Classical Latin Dictionary - this is essentially the version of Lewis & Short offered by the Perseus project
Ramminger's Online Neo-Latin Lexicon - regularly updated, the most extensive Neo-Latin dictionary available
Du Cange's Dictionary - an extensive dictionary of medieval Latin
Packhard Humanities Institute - huge word-searchable corpus of classical Latin texts (use to identify citations)
Capelli's Dictionary of Latin Abbreviations - especially useful for reading manuscripts, but also early printed books
Lexicon Morganianum - a lexicon of modern Latin terms compiled by David Morgan and maintained by Patrick Owens
For access to Irish Latin texts online click here.
A very large number of texts may be found freely available on Google Books, in the Gallica collections of the Bibliotheque Nationale, in the online collections of the Bayerische Staatsbibliothek, and in numerous other online library collections.
Other sites of particular value for Neo-Latin studies are: