We used Less for a project, but ended up moving to Sass: http://sass-lang.com/

The Sass compiler was (is?) more mature, and the language feels more consistent (see the '&' operator in Sass vs the :hover special case in Less). The editor/tool support was quite a bit better too.

Sass doesn't have a javascript in-browser implementation (afaik), but for rapid development it can watch a folder/file for changes and automatically recompile to static CSS files.

Plus there's a firebug plugin, so you get Sass line numbers / references instead of compiled CSS line numbers: https://addons.mozilla.org/en-US/firefox/addon/103988/