SubEthaEdit
I hadn't been paying too much attention to this Macintosh file editor, as its salient feature seemed to be collaborative editing, something that, while interesting, I can't seriously imagine wanting to use myself. Well, I recently heard tell of some of its other nifty features, and I thought I would try it out.
Now it's my main editor for code writing, at least. It comes with syntax coloring for a host of languages (inlcluding python), with more syntax plugins available here. It has a function popup to navigate your code; it's a Cocoa program, so I can use TextExtras, services, etc.; it can track and highlight recent changes you make to your file.
SubEthaEdit implements HTML preview like no other editor I've seen: it pops up a browser window within the editor that updates its display in real time as your type your source. The display uses the same webkit libraries as Safari, so the result you see will be just as seen in that browser.
Finally, SubEthaEdit is free for noncommercial use.