Dash is an API Documentation Browser and Code Snippet Manager. Dash stores snippets of code and instantly searches offline documentation sets for 80+ APIs (for a full list, see below). You can even generate your own docsets or request docsets to be included.
I've only just scratched the service of what this app can do but immediately it excels in documentation searches for Less, Node.js, jQuery, and another 75+ languages. Being offline means its always there for you and damn fast! Another nice feature, though I've not had time to try yet, is that it's customisable too so you can add your own docset via Javadoc-generated documentation, source files, any HTML Documentation, and the list goes on.
Best of all its free on the Mac App Store with no limitations but it will remind you to purchase from time to time (and its well worth the £5.49)