Everybody I know has probably heard me mention QuickSilver at some point. No other application exists that has as much buzz, while being nearly impossible to describe. While you could just install it and poke around, I'll try to outline some cool things that you can do with it, and as I find ways to make my workflow more efficient, I will share those tips with you.
The first thing you need to do is download QuickSilver. Download it from here and install. The preferences menu is where you will create an infinitely complex program -- eventually -- but for now just change the HotKey Activation to something easy, like Command-SpaceBar.
The most basic function of QuickSilver is an application launcher. Invoke QuickSilver, type the first few letters of the application you wish to load. If the application you wanted did not show up, press the down key until it is highlighted. If you want that application to show up first, by default, the next time you enter those letters, click the radio button beside the application name. Press Return to launch the application.
You will notice that second panel, beside the application name, displays the "Open" action. While this is useful (I use it all the time), there are many other actions within QuickSilver. But we'll leave that for next time. For now, enjoy the launcher.