Depending on how you lost the data:
Diskwarrior by Alsoft is an excellent utility. This works great if you've had trouble by incorrectly un-mounting a firewire drive, or if files that used to be on your Desktop have mysteriously disappeared.
Data Recovery Pro by ProSoft Engineering is supposed to be a great tool for data recovery, but I've never gotten to recover anything for me.
If the stuff has been deleted, and you're trying to get it back, this can be quite a pain.
Stay away from Norton anything. Their stuff has sucked on Macs for years, and maybe there is a reason they stopped making it. Their Anti-Virus stuff works OK.