The September 6th issue of PC Magazine has a very good article on loading up your USB Key with portable applications. I have several of these already, my favorites being Portable Firefox and RoboForm. With the rising popularity of the USB drives, portable apps could really explode soon.
Here is a quick list of what I use on my SanDisk Cruzer Micro:
I also have the following loaded but have not had a reason to use them yet.