There's some great suggestions above. Personally, I loaded the following apps onto my machine right away:
Fantasic app that makes it VERY easy to access anything on your machine. Though spotlight, which is integrated into the OS, covers some of Quicksilver's features, so I use that as well very often).
Decent maintence app that let's you clean up your machine regularly and run maintence tasks. I also have Onyx, but use this most of the time, as it loads up MUCH faster (doesn't verify the drive on every load, like Onyx) and has most of the same features.
Nice all around web-browser.
Video conversion/compression app. Nice and simple to use. Must have if you have an iPod, or post videos on Youtube.
In addition. Here's a few things I did right away that I think make the computer very easy to work with:
--Drag the applications folder into the right-most portion of the Dock to create a nice easy way to get to all your apps. Do the same for your HD, good shortcuts to have there.
--Set your expose mouse triggers, so when you hover into different corners of the screen it will: show the desktop, show all open windows, show dashoard, ect.
Hope that helps.