California Classical Association (North)
The Society for Classical Studies (formerly the American Philological Association)
North American Research Group in Ancient Myth and Mythography
BREPOLiS (databases available from Brepols) [subscription required]
Tesserae (a web interface for exploring intertextual parallels in classical poetry)
The Greek Transcoder (for converting old Greek fonts such as SGreek to Unicode)
The Alpheios Project (integrated tools for Firefox that enable Latin and Greek dictionary-lookup on almost any site)
Diogenes (an open source program for searching the TLG and PHI databases)
Linux Libertine (an open source, high quality font with very extensive Unicode coverage including an almost complete polytonic Greek set)
Cardo (another free and attractive Unicode font, designed specifically for Classicists, so the Greek and even the metrical coverage is almost complete, but other areas of the font table can be a bit sparse)
Junicode (one more free and stylish Unicode font, primarily for Mediaevalists, but with a beautiful old-fashioned Greek character set; currently the extended Greek set is patchy, however)
SC Unipad (a great and customizable Unicode text editor for PCs; feel free to download my preferred Greek keyboard layout for SC Unipad, based on Betacode and SGreek encoding [right-click or command-click, save-as; you will need to load it from within the program, under the menu Keyboard > Load])
Niklas Holzberg's bibliographies on ancient literature
Pinax Online [no longer online (RIP Marc Huys); link via Internet Archive Wayback Machine]
Dickinson College Commentaries
Carlo G. Carlucci's A Field Guide to Greek Metre (PDF, 2.9MB), posted with permission — also a review of the Guide from Classical Journal 75.2 (1979–80).
The Database of Greek Animal Sacrifice
The Ancient World Mapping Center: Antiquity À-la-carte
The Silver Muse (hypertext system of bibliographies, commentaries, etc. for Imperial Roman poetry) [no longer online; link via Internet Archive Wayback Machine]