
According to its producers, SubEthaEdit is "the only collaborative one you can actually use", including advanced features such as overhauled printing, exporting to HTML, both with collaborative metadata, completely customizable syntax highlighting, as well as a feature that some advanced users will be very pleased to see included - a UNIX command line utility.
SubEthaEdit 3.1 requires at least Mac OS X 10.4, and a license is priced at about $44. Since you can easily see other people connected to a certain document (this is what "friendcasting" is all about), as well as interact with them using iChat, if you're using Leopard, of course, I think a lot of you will be pleased to find out about SubEthaEdit.
...oh, yeah! A free 30 days trial version can be downloaded from here.