Lifehacker did a great little write up of 20 essential applications on your Mac. I generally agreed with their opinions. A few in particular that I find very helpful are ...
Disk Inventory X - This application is great for sniffing out large files on your hard drive you didn't realize are there. Before installing Leopard, I found 8 gigs of sample garageband song files that were easily moved onto an external drive.
Growl - Don't question it. Just install it.
TextExpander - This application is a huge timesaver. The trick is to remember it is there and to keep updating it with new snippets. I've been using the new version that came out a few weeks ago and had ho problems with it in Tiger or Leopard.
A few of the applications Lifehacker recommends don't really impress me so much. I prefer mail.app over Thunderbird and Safari over Firefox. Anyway, you can read the full article right here.