TextMate is a code editor for Mac OS X that focuses on being a reliable editor for both advanced and new scripters. If you download it, be sure to get the 2.0 beta version, as this one does not require a licence and and is made for the modern releases of OS X (10.7+). If you are unable to get the beta, for example if you have OS X 10.6, only trials are available for TextMate 1.x.
You can find a download on their website here.
The website looks somewhat outdated, however the program looks and feels very modern. It has features similar to Notepad++, like tabs for recent documents, language syntax highlighting and restore opened documents on launch. It can also give suggestions for code implementations as you write, taking into account what has already been written.
Here is a preview of how it can look.