The popular Bisigi themes are available for Ubuntu 10.10 Maverick Meerkat in a testing PPA. They will be included in the stable PPA once Ubuntu 10.10 will be released.
Add the Maverick PPA and install all the Bisigi themes (they include GTK, Metacity and icon themes as well as wallpapers for each theme) using the following commands:
sudo add-apt-repository ppa:bisigi/dev sudo apt-get update sudo apt-get install bisigi-themes
You can also install only a certain theme by replacing "bisigi-themes" with a theme name (see all the themes here).
If you're still using Ubuntu 10.04 Lucid Lynx, install the Bisigi themes from the stable PPA as usual.