I'm a devoted Quicksilver man myself, but the venerable LaunchBar seems to have started the whole keyboard launcher trend and retains many committed fans. Just updated to version 4.3, Objective Development has added a number of new features. In particular, Instant Open allows you to execute a command without hitting return, while Instant Send allows you easily to send files to LaunchBar (both of these are possible with Quicksilver, though the implementation is different). There are other improvements to the calculator, iTunes support, and more.
While I can't imagine switching myself, I've heard some on the Mac web suggest that LaunchBar is more stable than Quicksilver and so might be worth it for some. LaunchBar remains $19.95 and a demo is available.
[via Daring Fireball]