Update (24.03.2011): There is a much more up-to-date list here
with all the new themes.
Here's a list of some of the existing themes around:
X10 Res Mods
Transparent Status Bar Theme
HTC Sense and Stock Android Status Bar
There's also a lot more here
thanks to gavriel18
If I have missed any please PM me or post here and I'll update the list. I just scanned the first 3 pages in the forum.