LESS.app For Mac OS X
incident57.com/less
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
{Less} extends CSS with variables, nested rules, operators and more. If you're still building websites without it, you're an idiot. This app makes it dead simple to use {Less} by automatically compiling *.less files into standard CSS.