Buttons – A CSS Library For Creating Buttons
Buttons is a CSS library for creating highly customizable, flexible and modern web buttons.
It is built with Sass + Compass and has support for square, rounded or circular buttons that can be flat or not and having custom effects (like glow).
The sizes, colors, effects and fonts used can all be changed with the help of variables and it can be extended easily.
P.S. A Ruby Gem-friendly version exists as well.