jEdit is an open source Java-based text editor that has been around for years. On the positive side it's free, reasonably quick, and most importantly, it has a ton of third-party plugins. All those plugins are what keeps me coming back to look at jEdit from time to time.
Given those positive traits you'd think I'd use jEdit all the time, but no, I don't. Personally, in this age of AJAX Web 2.0 applications and Filthy Rich Clients, the jEdit user interface looks very dated. Beyond that, jEdit doesn't make many accommodations for the Mac OS X world, so things like opening and saving files, the menu system, and the dialogs look out of place on a Mac.
If all those UI woes don't bother you, and you can just look inside the editor and enjoy the big plugin universe, jEdit can be a decent editor for you.
In my case, I can't get past the UI. But I will say this: If I were to ever work on an open source project that I didn't create myself, and they would let me work solely on the user interface, jEdit would be at or near the top of my list for volunteer efforts. The vast plugin support for this editor makes it very tempting.
If you're a vi/vim user, there is a version of vim for Mac OS X, but even as a regular vi user, I was not a big fan of this editor. If you've never used vi before, I can't even recommend that you start down that trail, unless of course you're a Unix or Linux administrator, as you should know vi very well.
Beyond that, I found this vi implementation to be very slow on large files, and with no updates since 2007, it seems that this project may have been abandoned.
This has been a surprising and refreshing experience, and I've picked up several new tools for the tool chest. Komodo is my new editor for CSS, HTML, and possibly other language files like PHP, Perl, and Ruby. And Smultron and TextWrangler are battling it out for my new plain text editor of choice.