Quicksilver is a launcher for OS X. It's primarily used as a replacement for Spotlight. For alternatives see here.
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 for creating tasks and vast extensibility via plugins.
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Pro A lot more powerful than Spotlight
Pro Makes it really fast to do a lot of actions
There are customizable triggers and capability to execute multilevel commands - e.g. launch terminal app, give the location of script and supply parameters for the script with one command.
Pro Vast extensibility via plugins
A lot of different plugins for browsers, file systems, mail clients, etc.