Quicksilver

This is a superb freeware utility for your mac. It allows users to use the keyboard to quickly perform tasks such as launching applications, manipulating files and data, running scripts or sending e-mail. Mainly, I use it to launch applications in a jiffy... but this piece of software is flexible enough to cater from beginners to expert users.

Download it at http://www.blacktree.com/ (it's free), I strongly believe it'll be one of the mac tools you can't do without in the future.