Alfred is a productivity tool for OSX that provides numerous improvements to the operating system.
Quicksilver is an open source launcher utility for OS X that allows you to perform common tasks with only a few keypresses. It features an easy-to-understand noun-verb paradigm f...
LaunchBar was the first application available on the Mac (actually it dates back to NeXTSTEP times). It's a far more powerful replacement for the default Spotlight.