Google Fonts
Google Fonts is the world’s best Web Font service. Offering an intuitive and robust collection of open source designer web fonts. By using their extensive catalog, you can share and integrate typography into any design project seamlessly—no matter where you are in the world.
Related Tools

Alternatype
Alternatype helps you find free fonts with similar features to the most famous typefaces.

CSSans Pro
CSSans Pro is a colorful, sassy, CSS font created by Izabela Andronache and Codrin Pavel. You can use it for free on your next project.

DaFont
DaFont is an archive of free downloadable fonts. You can browse by alphabetical listing, by style, by author or by popularity.

Fonts in Use
Fonts In Use is a public archive of typography indexed by typeface, format, industry, and period. Perfect for inspiration and font selection.