CSS Juice has an interesting collection of rounded corner techniques to share:

Rounded corners is one of the most popular and frequently requested CSS techniques. Even the father of Internet Google also launch the rounded corners style markup with their Google Adsense and let users to custom their ads recently.

Actually, there are a lot of methods and techniques to create rounded corners with Cascading Stylesheets. Some are quite simple just need pure CSS, and a part of them need 2 to 4 background corner images and Javascript.

Find them here.