I really enjoy Dash Documentation Viewer . It's become entirely indispensable to me, and I use it dozens of times each day. It's the single reason I don't do all my coding on ubuntu instead of OSX.
It's a really polished documentation viewer that stores docs for almost any language or framework you can think of on your local machine, for lookups at the touch of a hotkey. It makes it possible to work even when the internet connection is sketchy or non-existent, and even when internet is good, it's far quicker than google for finding the information you need. It's even possible to pull down a local copy of stack overflow answers through it, though I've personally not used that particular feature myself and cannot vouch for it.
It has a built in ability to get the docs for every gem in rubygems.
I could go on and on, but please, try it for yourselves.