Vienna a lightweight RSS reader.
Adium Multiprotocol IM client supporting AIM, Gtalk, Facebook, etc. You can also customize the hell out of it.
Notational Velocity Very lightweight note taking app.
Mindnode Excellent mindmapping app. Try the free version.
Vox Music player with support for other file types. Looks prettier than VLC, functional enough for me.
AppCleaner You may think you don’t need this, but an uninstaller app is good to have on your mac.
Dropbox Absolutely essential. Free, secure, online backup.
Handbrake Video converter/DVD ripper app, can pretty much handle any video file you throw at it.
Virtualbox If you want to geek out with virtual machines or linux distros.
Caffeine. Sits in your menubar and stops your computer from going to sleep. Very useful when watching movies, etc.
Transmission Probably the best torrent app for mac right now.
Growl A great notification engine, one of those apps you wonder why OSX hasn’t copied it yet.
Some of my favorite paid apps (and worth every penny are)
Pixelmator. A photoshop alternative, with a beautiful UI and loads cheaper.
Littlesnapper Might not be up your tree, but one of the more beautiful and useful programs I have on my computer.
Shovebox Little information collecting app, sits in the menubar and waits for snippets, pictures, whatever.
You may also want to check out Macheist. Every year or so they have a large shareware discount bundle, and some free (usually paid) apps that go along with it.
If I had to pick 3 or 4 essential apps for my computer, I’d go with Adium, Dropbox, Growl, and Shovebox. I use all of them every day, and are the first things I installed on my computer.
Crap, that was long.