The Less add-on provides a helper which enables you to use Less files from your add-ons or theme. Simply reference a Less file, and it will be compiled to CSS for you. Makes changes to your Less file and the CSS gets re-compiled.
Compilation takes place on the server using a PHP-based Less compiler (written by Leaf Corcoran), so there is no need for less.js on the client, or any node.js on the server. Simply load the helper, link the file, and you're away.
More information on this add-on is available on my blog.
More information about Less is available here.