Quicksilver (and link to new manual)

If you use a Mac – you will definately want to check out Quicksilver. This free and soon to be open-source software will change the way you use your Mac. In a nutshell, Quicksilver allows you to launch any application, folder or file by simply typing in part of the filename.

It acts as simple personal search engine for every file on your system.

Initially, Quicksilver was launched without any documentation, but now the fine people at Blacktree Inc. have released a comprehensive Quicksilver Manual.