Chromifox looks very good too, but it’s not original. Plus, it doesn’t integrate well with the Aurora GTK engine/theme.
Here is a screenshot of rein in action on my Xmonad + Vimperator setup (with Terminus as the GUI font, of course).
You gotta love those tabs. Finally, a theme that looks clean, simple, and unique.
UPDATE Feb 17, 2011: Apparently, people are really intrigued by this post, even after two years. Since the time of this post, I’ve swtiched to the Devious Green theme for a good while, but now I just use the default theme (i.e., no theme), because my default GTK theme is already very clean and minimalist (the Mist theme that comes with the basic gtk-engines package on Arch Linux). Oh, and I’ve since switched to pentadactyl instead of vimperator because the former is more actively developed.