TextMate is a popular text editor for Mac OS X.   It is developed by MacroMates http://macromates.com/.  It comes with code markup for most programming languages, and a built in integration with subversion and X-Code's compilers, and it looks sexy.  It is highly extensible with macros and "snippets" which are like shell expansions. 

TextMate is kind of like Emacs and OS X had a baby.

TextMate is open source (but you are still required to pay for a license)

It is released under the GPLv3 (but patches must be PublicDomain so the author and his friends can use them in ClosedSource projects)

http://arstechnica.com/apple/2012/08/textmate-2-0-goes-open-source-in-response-to-os-x-restrictions/

http://arstechnica.com/apple/2012/08/odgaard-i-will-continue-working-on-textmate-as-long-as-i-am-a-mac-user/