I use NUnit. A lot. Almost every time I hit compile, in fact. And I'm not a big fan of the command line version; there's a certain feel-good factor from seeing all of the orbs turn green. But this is where my problem lies with NUnit - although it's technically great, the interface just looks really clunky. Half of the problem is that it doesn't support Windows Themes - the controls neither have their FlatStyle set to System nor has the call to EnableVisualStyles been made. The rest of the problem is just to do with small tweaks around changing text to images, replacing the red/amber/green orbs with prettier ones, and so on.
So, as I've got an idea for another NUnit related project anyway, I decided to get to grips with the source code for it by refreshing the UI. Below are before and after pictures, and the updated files for running it yourself are here:
http://www.altervisitor.com/software/NUnit_GUI_Update_Release001.zip
As I've not changed any real code, only the UI of the EXE (and a UI DLL), this archive simply contains two files to overwrite those that already exist in your NUnit folder. All your current existing projects, links, etc. should continue to work. Note that this is an update to the latest version at the time of writing - v2.2 (v2.2.0) , and will only work with this version.
If you like/don't like this update, please get in touch, as I'll be mailing the NUnit guys themselves over the next couple of days to see if we can't get the proper project updated.
Within the next couple of days I should have finished the NUnit-based tool I'm writing, too, which should be well worth the download...

Original NUnit 2.2 GUI

Updated NUnit 2.2 GUI