There is a long and short answer to that. Short: nothing as of now that even comes close to Unsanity.
Long: There has been people editing themes on Leopard using system files and package installers, not nice little apps, for some time. Some mod the new CoreUI files, ArtFile.bin and SArtfile.bin, which include a different encoding technology than the old rsrc files or Tiger and are much harder to theme. There is an easy Terminal command that allows Leopard to use the old rsrc files for easier themeing but it is not perfect and somewhat limited. A good list of available themes is here:
http://macthemes2.net/forum/viewtopic.php?id=16788321
There is a new duo of applications that are coming out, Architect and Façade:
http://www.slightlypretentious.com
These promise to make very, very easy and make applying theme even easier. As you can see from the link, Architect is in beta and works fine, but the themes made cannot be applied yet without the not out yet Façade. So there is hope, just in time for Snow Leopard, and considering our luck, it will be using vector UI
Peace. Also, I would look around Macthemes forum, there are a lot of little things you can do, such as change the icons and the dock, wallpaper, app skins, the works. Have fun!
