Firefox Portable is an awesome piece of software. Available in the same versions as the now World-famous Firefox Browser. This release has been adapted to run from a USB Pen-drive, leaving no files on the host system.
A brilliantly simple idea, but hugely useful. I am spending a lot of time using the public access PCs in the library at Uni this term, since I live off campus and have time to kill between lectures. For some reason, unknown to me, the IT management at the uni don't have Firefox installed on the library PCs. This means that I would be forced to use Internet Explorer 6 if it wasn't for Firefox Portable.
An added advantage to Firefox Portable is that all your cookies, favourites, and even Firefox extensions are stored on the USB drive along with the program itself, so you will no longer need to log in to forums every time you use a different PC, or try to remember all your favourite URLs.
The one disadvantage of Firefox Portable is the fact that it is running of a Pen Drive, making it inherently slow, even on a USB 2.0 drive. However, once it is up and running it's usually fine. The application is pleasantly small when installed, a mere 16.3MB so even small pen drives should cope, and with 1GB available for under £15 it's quite a practical idea.
Firefox Portable, along with a number of other handy Portable applications are available from portableapps.com.