I’ve been a fan of the Clarity Icon Theme for a while now. I’ve used it in Linux Mint and Kubuntu, but I recently installed it again on my lovely Xubuntu/Voyager 12.04 desttop.
The icons are similar to the AwOken Icon theme, which I’ve also used in the past (there’s a KDE version, also); but at the moment I think Clarity is my favorite. Both Icon themes are fairly simple and minimalistic. They are both quite versatile, blending nicely with many different desktop wallpapers (why is it called wallpaper if it’s on a desktop?) and themes. And Clarity and AwOken can be made to use different colors. The orange goes especially nice with Ubuntu/Unity themes.
Faenza is also a wildly popular Icon set for Linux and is used as the default by several Linux distros. The square, button-like icons have a nice uniformity and look good in Dock launchers and in the Unity Launch Bar as well. But sometimes I don’t feel like having multi-colored icons in my dock (I use Docky) and when I’m in the mood for sleek, attractive icons that aren’t too distracting, Clarity or AwOken are a good choice, no matter what Desktop Environment you prefer.