I've written about QuickSilver as <a href="http://www.examiner.com/apple-products-in-national/my-top-five-free-mac-... of my Top 5 Favorite Apps</a>. It's been updated by the new open source developers behind the launcher. You can now <a href="http://www.qsapp.com/download.php">download</a>, at no charge, versions of QuickSilver for Lion, Snow Leopard, Leopard, Tiger and Panther. The Snow Leopard version has a couple of nifty new features and many bug fixes. Principal among the new features is an automatic plug-in updater.
If you're not a QuickSilver fan, you might want to check out <a href="http://www.alfredapp.com/">Alfred</a>, another free app launcher for Mac OS X 10.6 Snow Leopard and 10.7 Lion.