Spice Up Your Desktop with Cinnamon!
If you are disgruntled by the new interfaces provided by recent distribution releases, namely GNOME 3 and Unity, you might want to take a look at Cinnamon. With its traditional feel and extreme theme-ability, Cinnamon is a desktop interface bound to spice up anyone's computer. The general feel is that of GNOME 2, or perhaps XFCE, but its polished look and downloadable themes make it truly exciting to behold.
(Screenshot from http://linuxmint.com.)
Development has revolved around Linux Mint, but Cinnamon also can be installed on Ubuntu for those so inclined. If you've never seen it in action, be sure to give it a whirl: http://cinnamon.linuxmint.com.