If you use Emacs, markdown-mode.el offers a mode for Github-flavored Markdown.
(If you don't use Emacs, you're probably not well advised to pick it up just for this sole purpose. It isn't so much a text editor, as a virtual Lisp machine in which has been implemented a text editor whose conventions are quite unlike those of any other such tool; Emacs in fact has its own standard library, which in the current release (version 24.3, March 2013), weighs in at 172M of source. There is nothing you can do in any other text editor which you can't do in Emacs, often more quickly and efficiently; the trade-off is that, depending on your purpose, you will first need to spend anywhere from several days to several years first acquiring expertise in the use of Emacs.)
I had hoped the Mou developer would show the good sense of implementing a reasonably general parser which could accept a language specification, in order that his code could eventually support more dialects of Markdown than just the canonical one. Were that true, it would be trivial to modify Mou for Github-flavored Markdown -- but, regrettably, on examining the app bundle's contents, I find this appears not to be the case.
Covering what I understand to be the popular OS X editors, TextMate can apparently be made to support Github-flavored Markdown. I gather there is also a Github-flavored Markdown plugin for Sublime Text's newer versions, although why anyone would want to pay $70 for such a fundamental capability as text editing is beyond me. BBEdit doesn't seem to have a module, or at least I can't find one via Google, but does anyone use BBEdit any more anyway? And, finally, you'd really expect there would be a plugin for Xcode, but I haven't been able to find one for that, either.
And, finally, examining a slightly different approach, Marked is an OS X-native Markdown previewer which has Github-flavored Markdown parsing built in; the way this works is, you edit the Markdown source in the editor of your choice, and Marked updates its rendering of the file to show you what the result will look like. I haven't used it myself, but it looks like it might be of use. (Sure, it costs money, but $4 won't even buy you a deck of smokes or a gallon of gas these days, so it's not like it costs enough to care about.)