If you use IM a lot, you'll want Adium
http://www.adiumx.comIf you dont want to use Time Machine for backup, either Carbon Copy Cloner or SuperDuper work great.
http://www.bombich.com/software/ccc.htmlhttp://www.shirt-pocket.com/SuperDuper/Sup...escription.htmlAppFresh is a good way to help keep all your applications up to date, but its still in beta so use at your own risk.
http://metaquark.de/appfresh/If you have an iPod, or if you would rather keep your DVD's on your computer, Handbrake makes ripping and converting them to compatible video files a breeze.
http://handbrake.fr/You'll definitely want Flip4Mac to be able to play Windows media files through QuickTime
http://www.telestream.net/flip4mac-wmv/overview.htmAnother great media player thatll play just about anything is VLC
http://www.videolan.org/vlc/If you use a USB drive or MP3 player between your Mac and other PCs alot, HiddenCleaner is a great way to get rid of all the hidden files OS X tends to leave behind
http://www.versiontracker.com/dyn/moreinfo/macosx/34318SimplyBurns is a great CD/DVD ripping/burning utility
http://simplyburns.berlios.de/You'll probably want StuffIt for extracting zips, rars, etc.
http://my.smithmicro.com/mac/stuffit/index.htmlThe rest of this list either costs money, or is completely pointless but still fun to play with
AppZapper-Deleting apps in OS X is pretty straightforward, but they almost always leave behind something that you dont know. This app scans your hard drive for those unknow files and sends them to the trash. You can find free alternatives to this.
CandyBar-This is THE app you want if you like to customize your system/application icons.
CoconutBattery-This is a great app to track your laptop battery's life
Little Snitch-This here acts like a reverse firewall. Instead of monitoring and blocking whats coming into your computer, Little Snitch monitors whats going out. When running, it tells you when an application is trying to access the internet, where its going and on what socket. You can tell it to allow or deny access to it.
BootXChanger-Lets you change the grey Apple image you see on startup.