Linotype - Linotype: Fonts for Desktop, Web & More
What Font Is - Find any font from any image (commercial or free)
Google Fonts - Making the web more beautiful, fast, and open through great typography
FontFace ninja - A browser extension to identify and try fonts on any website
Fonts.com - Fonts.com provides the biggest and best selection of high quality desktop and web fonts. Try, buy and download classics like Helvetica, Univers, Frutiger, Trade Gothic and Avenir along with popular new fonts like Soho and Neo. Find your type!
Fontsnatcher - Fontsnatcher helps you in identifying, testing and discovering new fonts and typefaces being used on the internet.