Easy to use

http://twitter.github.com/bootst - Twitter Bootstrap

http://foundation.zurb.com/ - Zurb Foundation

Typography

http://www.google.com/webfonts - Google Web Fonts

https://typekit.com/ - Typekit

http://letteringjs.com/ - LetteringJS

http://www.kernjs.com/ - KernJS

http://www.dailyjs.com/ - DailyJS

http://compass-style.org/ - Compass framework

http://lesscss.org/ - CSS Preprocessor Less