From the portable apps website:
What is a portable app?
portable - carried or moved with ease
app - a computer program like a web browser or word processor
A portable app is a computer program that you can carry around with you on a portable device and use on any Windows computer. When your USB flash drive, portable hard drive, iPod or other portable device is plugged in, you have access to your software and personal data just as you would on your own PC. And when you unplug the device, none of your personal data is left behind.
No Special Hardware - Use any USB flash drive, portable hard drive, iPod/MP3 player, etc
No Additional Software - Just download the portable app, extract it and go
No Kidding - It's that easy
There are a number of Portable Apps that are free. You might want to start with the Portable Apps Suite.
You might also want to check out Snapfiles and the Opera USB browser (I have used the portable version of Mozilla Firefox, have not tried Opera's USB browser yet).