You may have heard of LESS. It’s a Ruby Gem that allows you to write CSS files with variables, nested rules, operations, and “mixins,” which are akin to custom functions in a programming language. The LESS files are compiled into pure CSS before going into use.
WP-LESS is a plugin that, using the lessphp library, brings LESS to WordPress. I have yet to try it for myself, but it certainly sounds intriguing.
The plugin watches for edits to your LESS source code, then recompiles and caches it as plain CSS, which is served up to your visitors.