http://gnome-look.org/content/show.php/GnomishGray?content=149193&PHPSESSID=4d672c87c3867429f5197e1ab6ab7851. Gnomish Grey. An excellent GTK 3.0 theme.
http://gnome-look.org/content/show.php/Zukini?content=147431. Zukini. This is another very cool GTK 3.0 theme. This may not work with Gnome Shell 3.5, so if you are running Ubuntu 12.10, you are out of luck. But it also includes a theme for the Awesome Window manager, so that is a nice bonus.
http://gnome-look.org/content/show.php/Orion?content=148527. Orion GTK 3.0 theme. A light coloured theme, this will suit any desktop user.
http://gnome-look.org/content/show.php/GrayDay?content=149310. Grayday. I love this theme. It comes highly recommended.
http://gnome-look.org/content/show.php/MediterraneanNight?content=148398. Mediterranean Night. Another grey theme with various levels of darkness to suit any user. This theme has a very high rating and is highly recommended.
If you are running the Ubuntu Unity desktop and you want to add a nice custom skin to it, follow the instructions here. http://www.omgubuntu.co.uk/2012/03/how-to-easily-skin-ubuntus-unity-desktop/.