How did I manage to miss this for so long? TextMate, the Mac text editor that set the standard by which all modern programmer’s editors are judged, is now Open Source. TextMate 2.0 is still in alpha status, but the first alpha build was released over 21 months ago, so it’s probably pretty stable by now.
You can download the binary, or access the source on GitHub. The latest builds require at least OS X 10.7, so those of you still running on older versions are out of luck. On the bright side, it costs less for an OS X upgrade than it used to cost for a TextMate license.