Quicksilver
Blacktree’s Quicksilver is very neato. There isn’t a lot of documentation on the website [that I could find], particularly regarding what Quicksilver does.
In a nutshell, Quicksilver is a command-key-activated launcher. Hit cmd-space, type ‘CALC’, hit enter, and it will launch Calculator.app.
What makes Quicksilver truly great is its ability to catalog all of your applications and their preferences, your favorites, your contacts, and so on. This means that Quicksilver isn’t restricted to launching your apps — type in ‘UNXM’ and it will open your favorite website. If you type the first few characters of one of your contact’s names, you can quickly pull up the Address Book page for that contact.
Quicksilver is a great app, and at the low price of zero, you can’t beat it.