CSS3 Buttons Set (Simple Markup)
Collection of buttons that use the simplest possible markup and shows a variety of CSS3 styles in different combinations. It is made and maintained by Chad Mazzola.
In IE9 and below the buttons fall back to the default button. Although you can use ColorZilla to generate the CSS code for supporting the gradients in IE6-IE9.
The CSS Code is well written, organized and commented. So study the code, adjust it, play with it, build on it and share in the comments if you come up with something really nice. Got questions? Ask in the comments!