I'm a LaunchBar user myself, although I have used QuickSilver (and I know many people prefer it for it's extensions abilities and the fact it's freeware, I switched mainly due to LaunchBar's speed and stability), and I've also used AppRocket on Windows (although was fairly let down by it's scope and capabilities); recently the MSN Toolbar Suite Beta became a valid alternative due to the added launching feature.
Whatever the implementation, one-stop keyboard based applications launchers are the thing for accessing the files and applications on your system with the greatest of efficiency, you can't beat them.
The reason I mention this is because of the newest entry into OS X launchers is RocketLauncher, which appears to resemble LaunchBar very closely, but with a big difference: it isn't just freeware, like QuickSilver, it's open source, which even more greatly opens up the possibilities for extension and improvement. Take a look, soon it could get very interesting (although more so come the release of Tiger, and the introduction of Apple's Spotlight).
Anyone know any good GUI launchers for the Linux/*BSD/Solaris/QNX world?
CLI doesn't count, I could do that on OS X (and spend 50% of my time doing that, but everything else you need a completely different beast, one which as I just said, meta-data file systems and indexing, ala Reiser, will make much more interesting).