This seems like a very simple idea, but it is very handy. I have a USB Keychain drive to hold installers, updaters and tools that I use when working on other people's Macs.
One of the advantages of OS X is that, as programs are usually in standard locations, you can use a single alias to open, say, Terminal on anyone's machine. Just make an alias to your copy of Terminal (assuming it's in the usual location) on your portable drive, and then when you plug the drive into someone else's Mac, the alias will open their copy of Terminal. Lovely.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20021205061123125