New Wave is a truly awesome Gnome theme available in Ubuntu's community-themes package for quite a few releases.
Some community themes from the Ubuntu Artwork Wiki made it into the "community-themes" package from Ubuntu 10.04 Lucid Lynx (which you can install using: sudo apt-get install community-themes) however some amazing themes didn't.
Finestly Ying and Yang are 2 new proposed Ubuntu Lucid themes. Ying is a very dark theme and Yang - a very bright one, both using the Murrine engine and adjusted for the Humanity icons:
Ubuntu Sun is a new proposed theme for Ubuntu Lucid Lynx for which, up until now there was only an unofficial version available for download which is now called Ubuntu Sun X.
There have been proposed quite a few themes for Ubuntu Karmic Koala 9.10 and if you want to install them all in Ubuntu Jaunty for instance, you can just do: