From the homepage:
LESS extends CSS with dynamic behavior such as variables, mixins, operations and functions. LESS runs on both the client-side (IE 6+, Webkit, Firefox) and server-side, with Node.js.
On the Mac, there's Less.app. The homepage looks a bit like a release announcement from Apple. What it does is to monitor a configurable set of directories for changes and invokes the "lessc" compiler.
On Linux, the equivalent is a few lines of bash:
If you can't see the script embedded above, you can see the source here.