Google Fonts - Making the web more beautiful, fast, and open through great typography
What Font Is - Find any font from any image (commercial or free)
Open Font Library - The mission of the Open Font Library is to promote your freedoms as it relates to the use of type.
FontFace ninja - A browser extension to identify and try fonts on any website
Dafont - Archive of freely downloadable fonts. Browse by alphabetical listing, by style, by author or by popularity.
Fontsnatcher - Fontsnatcher helps you in identifying, testing and discovering new fonts and typefaces being used on the internet.