If you’re always having to use public terminals or friends’ PCs, you definitely would have been in a position where you wanted to or had to install an application to get some stuff done. In another situation, the applications might have already been installed on the machine, but the settings isn’t configured to how you want them to be, causing heaps of time wastage in reconfiguring it (and risking the wrath of your friend or sysadmin).
With USB thumb drive storage capacities skyrocketing in recent times, you can now have those applications you need stored in it, ready to be used right away with Portable Apps. The list of applications being made portable seems to be growing so there is a chance more applications will be added soon if the one you’re after isn’t listed. All portable applications are open source and seem to be updated fairly regularly.
However, with more and more traditional desktop applications being turned into web applications (e.g. Writely, Google Spreadsheets, Google Calendar, Meebo) as well as internet access being available almost everywhere, IÂ believe the only portable application worthwhile having on a thumb drive would be Firefox Portable since everything else is available through the web browser. Hmm, I think I should stop before I start contradicting myself..