I have just finished development of a TWiki and Foswiki markup editing package for Sublime Text 2. The project is hosted at GitHub: https://github.com/darryllawson/SublimeTWiki. See the README for installation instructions.
The main feature of the package is syntax highlighting, but it also features some handy auto-completed snippets and heading navigation via Command-R (OS X) and Control-R (Windows and Linux).
I have sent a request to have this package added to the Sublime Package Control repository. I guess that’ll take at least a few days to be actioned. For now, you can manually install it.
Why did I develop this? I use TWiki at work a lot, and using Sublime Text in conjunction with the It’s All Text! Firefox add-on, makes editing TWiki pages significantly less frustrating, and more productive. I actually created a similar thing for gedit back in 2011 (see this post), but I have long since stopped using gedit.
If you are not using Sublime Text, then see this post for some reasons why you possibly should.
Enjoy!
Update 23 Feb 2013: The TWiki package is now available from the Sublime Package Control repository.