PHP 5.1.2+ is required.
$ sudo apt-get install php5 # If PHP is not already installed, of course.
Basic usage is very easy:
$ watch-less /path/to/less/files /path/to/css/output
You can also specify the output extension (default ".css"):
$ watch-less /path/to/less/files /path/to/css/output .less.css
Note that this script is recursive and create directories if needed.
Enjoy :)