Evolve is a simple and minimal light theme designed to be easy on the eyes. It sports a metalic grey color and smooth transition effects. It was recently ported to GTK3.6 and a couple of updates soon followed.
Evolve includes a GTK2 theme using the murrine engine, a GTK3 theme, a metacity theme and a mutter theme. It also includes themes for Unity and Xfce.
The recent updates add active button styling, something that should have been there from start, restyled sidebar with less contrast, reworked GtkScale widgets and transition effects in buttons and entries. It also adds couple of new widgets like the GtkLevelbar. The theme looks best with Nautilus 3.6 :)
If you use Ubuntu 12.10, you can install Evolve from the PPA.
sudo add-apt-repository ppa:satyajit-happy/themes
sudo apt-get update && sudo apt-get install evolve-gtk-theme
For other distros, download Evolve from deviantArt.