Komposé is a task manager for KDE that generates thumbnails of your windows grouped by your current workspace or all workspaces. The old-school way to manage windows is to use a taskbar or to move through your windows by using Alt-Tab and Alt-Shift-Tab. I usually have 10 to 20 windows open on each virtual desktop, however, so this can get tedious pretty fast.
With Komposé installed, I can simply press Win-Tab (finally, a use for the Win key!) to view thumbnails of all the windows on my desktop and quickly select the window I want to switch to, rather than having to tab through them or try to find the proper taskbar entry. You can use the mouse to select the window you want, or use the Tab key and then press Enter to bring it into focus.
Since I started using Komposé, I've removed my taskbar on my main workstation. Not only is Komposé faster, it's helped save a bit of screen space, too.
Komposé in action - click to enlarge
The first time you run Komposé it takes a few seconds to draw the thumbnails, but it seems pretty speedy after that. The author does caution that the application's performance is heavily dependent on your CPU, so Kompos may not be the best choice for slower machines. I use it on an AMD Athlon 64 3000+ with 2GB of RAM, and it's quite snappy.
The Komposé package is available in Ubuntu/Kubuntu Breezy and Dapper; just run
sudo apt-get install kompose or use Synaptic to install it. If your distro doesn't have Komposé in its package repository yet, grab the source from the BerliOS project page and try it out. You'll need Qt 3.2, KDE 3.2, and Imlib2 to compile and use Komposé.