OS X Terminal "must have" utilities I've just read with pleasure, and learned a lot from these questions :


OS X Terminal Tips and Tricks
OS X Hidden features and nice tips &amp; tricks
OS X Applications you can't live without


But I didn't find in these some of my favorites tools, which I use on a daily basis, and I think that's because these are OSX/Terminal intended tools, not fitting well in above questions.

My favorites are, by order of preference: oh-my-zsh, homebrew (but also macports), janus...

These tools having in common :


Terminal: install and use
OSX Intended: brew is, git is not!
Easy Install: one liner, ie. git clone; wget ... etc.
Extensible: possibly has extensions or simple API (eg. oh-my-zsh with plugins, brew with formulas, janus with bundled vim plugins...)
Often open source, free, hosted on github, scripted, mostly (but not only!) with ruby...


Do you know any other similar must-have tools?
I would love to discover new ones!

Please only include one utility per answer. Also, please don't add duplicates. Check first before posting. If you see a duplicate, kindly downvote the answer and inform the author by leaving a comment.
Homebrew

The missing package manager for OS X.

It is an amazing package manager, very light and easy to use. KIS Principle, that makes me think of archlinux. Its community is big and very active.
(see also macports which brew doesn't replace completely, I prefer installing packages with brew, but some complex ones are only on macports)
See also brew tap command which provides some missing formulas that can be useful like when you need latest php version &gt; brew tap josegonzalez/php.
(Homebrew has a policy of not replacing system components, but hey, sometimes you have to)