Flash drives are pretty common these days. They are the easiest way to carry around files, and even to transfer smaller ones between two computers in the same location (that aren’t networked). If you carry one on your keychain, and have some free space on it, you might consider putting some useful apps on there, as you never know when one might come in handy.
TalesofInterest has compiled a list of some of the best portable programs currently available. Most of them are centered around fixing a computer that is under the weather, though you’ve also got web browsers, graphics editors and even games on the list. With these on your flash drive, you can make almost any computer feel like your own. The best part is that every one of them is free (only one has a “pro” version, but it isn’t necessary to use all of the features).