Nubbinator: what do you plan to do with your Mac?
Personally I don’t think any of the software listed so far is “essential” (I’m not sure if any third party software is “essential”, frankly). What you’ll want to install depends on what you plan to use it for, how much you need to customize the behavior of your computer and what kind of background experience you have, etc.
Also, are you looking for open source apps, free apps, paid apps, or what? Do you have a budget?
For the things I do with my machine, essential GUI apps include: Adobe Photoshop and Illustrator, FontExplorer X Pro, Adium, TextMate, NetNewsWire, Tofu, LimeChat, XCode.