Katapult is an application for KDE, written in C++, designed to allow faster access to applications, bookmarks, and other items. It is plugin-based, so it can launch anything that is has a plugin for. Its display is driven by plugins as well, so its appearance is completely customizable. It was inspired by Quicksilver for OS X.
Use alt+space and start typing - you'll be presented with an option to launch something depending on what you've typed.
For example, hit alt+space then type "a" "m" (hit the two keys - no spaces or quotes) and this should offer to launch amarok - hit enter to launch
If you have no notification-area icon - then hit alt+space then ctrl+c to bring up the case-sensitive menu to allow you to configure katapult
We now have a wiki - http://katapult.kde.org/wiki/