csspool-st: csspool:

• Compress css, but without support of any kind comment hacks. There are better ways to do crossbrowser styling. For example, IE conditional comments.
• Compression done with the help of real css parser (csspool). Not Regexps.
• Pure ruby.

• yuicssmin. Ruby. The YUICSSMIN gem provides CSS compression using YUI compressor from Yahoo. It uses the Javascript port via ExecJS.
• cssminify Ruby. CSS minification with YUI compressor, but as native Ruby port.
• css-compressor. The best of the best. P☣P :(
• yuicompressor. Java. Ruby wrapper: ruby-yui-compressor
• rainpress. Ruby. Seems to be dead ☠.
• http://friggeri.net/blog/a-genetic-approach-to-css-compression/