QuickSilver
QuickSilver, to put it simply, is an app for launching apps. Just a quick keystroke, type the first few letters of an app you want to launch, hit return, and BOOM! you're off! OS X just isn't complete without QuickSilver.
Growl
Growl is a system-wide notification app. This is another app that OS X isn't really complete with out. With a Growl-enabled app, Growl just works. When you first install a Growl-enabled app it is registered with Growl and can display notifications about that app. With an extension for Mail.app, you can see snippets of new mail that you receive in a Growl notification. If you're using Vienna as your rss reader, you can see how many new articles that you received on it's last refresh. Install Growl, you may not even know how many apps you have that already work with it.
VirtueDesktops
VirtueDesktops is an app that offers virtual desktops for OS X Tiger. If you have been keeping up with the Leopard news, you will know that Spaces is a virtual desktop program that will be included with the new OS. So, if you are still running Tiger, VirtueDesktops is the program for you. Unfortunately VirtueDesktops has been abandoned by it's developer, so it is left in it's beta form that can be a tad buggy, but it is better than other alternatives for virtual desktops.
Licensed
Licensed is an app that you can store and sort all of you license keys in. This app is very convenient, lightweight and easy to use. So, if you would rather store your keys on your computer, Licensed is for you.
Jiggler
Jiggler is an app that keeps your Mac from going to sleep or turning on the screensaver. What it does is jiggle the mouse at specified intervals set by you. I find this great for when I am reading articles on websites, and my screen doesn't dim because of Jiggler.
Caffine
Caffine does what Jiggler does only it lives in the menu bar instead of the dock. Personally, I prefer Caffine over Jiggler because it doesn't actually jiggle the mouse cursor.
Hex Color Picker
In short, the Hex Color Picker lets you view and edit the hexadecimal HTML color codes in the Mac OS X color panel. This can become very handy if you are a web developer or programer in need of a color hex code. The Hex Color picker can recognize short hand and HTML/CSS color keywords, and it has a live preview of the color as you enter the hex code.
AppTrap
AppTrap is the feature that Apple forgot. It is a simple PrefPane that detects whenever you put an application in the trash. When you do put an app in the trash AppTrap offers to delete associated system files that would normally get left behind if you just put the app in the trash. AppTrap is a quick, easy, and streamlined way to delete all of the files associated with an application.