csspool-st: 
- 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/