I spent a long time trying to find a Notepad++ equivalent for Mac OS. As recommended by others, I have tried TextWrangler, Smultron, and TextMate (the trial/free version, of course). Among the above mentioned, I liked TextWrangler the most. But in one way or another, they can’t match the same level of user-friendliness of Notepad++ on Windows.
This frustration led me to step into the realm of real programmers and start learning to use Aquamacs (based on GNU Emacs but has Mac-looking interface). For starters, it is certainly more difficult to learn than Notepad++. However, it is great for editing STATA do-files, TeX files, R scripts (which Aquamacs can run internally), etc. It is, in my opinion, also the best free TeX editor for both Windows and Mac OS. You could also use it to compare files, which comes in handy if you want to tell apart similar do-files. And these are just the tip of the iceberg of what Aquamacs is capable of. In short, Aquamacs will be worth your investment (of time) in the long run.
This note only offers a few useful links. If you run into trouble with any of them, or find anything difficult to follow, feel free to get in touch with me via email.
Set up Aquamacs: Download and install Aquamacs.
Getting started with Aquamacs: is essentially the same as Emacs. Therefore, almost all Emacs help files apply to Aquamacs. A thorough introduction by Jeremy Zawodny can be found here. The most useful chapters for me are 2, 4, and 5. Make sure you follow through the tutorials closely.
Integrating with Stata: unfortunately Stata does not come as a default Emacs mode. But thanks to Bill Rising and his ado-mode, this is made possible and simple. Please follow his tutorial here.
Integrating with LaTeX: Tex mode comes as default with Emacs, and made better with the AUCTeX extension here.