There's a bunch of really great apps out there. Some on the market, some not. Do you have any particular type of app your looking for? Here's a few that I use all the time. You should check them out.
LauncherPro Plus (not on the market you will have to visit here to download)
ADW
K-9 Mail (Much more versatile mail client)
WeatherBug
Beautiful Widgets
SetCPU
Titanium Backup
Xscope
Pure Messenger Widget