One of the best tools in my toolkit for working with SASS/LESS and combining JavaScript files (and more) is CodeKit. Just at the end of last week, the app was updated to a version 2.0 which adds some serious improvements over the previous version
In the new version, things such as built-in Bower support with thousands of components, better compilation and auto-refresh support have been added. This does make life quite a bit nicer when it comes to working with assets, especially with the new Bower support.
The new CodeKit version is very nice and after having worked with it for a few days now, I consider this both a must-have (like the previous version) and an easy upgrade at just $29.