Andy Peatling's project, CSS Tweak, is a fantastic online tool that can optimize your CSS files. Simply provide CSS Tweak your CSS file, either by pasting (temporarily disabled currently) it or selecting it to upload and configure the options to determine how tweaked you want your file to be, ie to remove comments, whitespace, etcetera. Hit the tweak button and you will quickly be greeted with your new, smaller file in addition to a percentage number of how much smaller it was able to make your file. When I tried it, one of my stylesheets was made 32% smaller with the other at 23%. This is great for cutting down on file sizes and loading times.
CSS Tweak is currently a pre-release, with upcoming versions hoping to tackle Internet Explorer CSS and XHTML markup. Go on and give it a try.