There are tons of different ways to get rounded corners on HTML elements using CSS, all of which require employing various image snippets. These are effective, but I found a nifty little resource called (what else?)
Nifty Cube.
Allesandro has put together and impressive, simple, cross-browser compatible (for now) way to create rounded effects on html elements with a tiny javascript and CSS. The files required are only 12 kb total and are so easy to use, they are worth looking at. He has great documentation and tons of examples of their usage on his website.
I applied the javascript on my master page for my current application and I don't have to change my images if my color-scheme changes or whatever...
At first I was skeptical, but when I saw the guys at TextDrive using is on their webmin site, I figured it has to be good :)
Nifty Cube