I keep a thumb drive on my keychain. It contains many portable-version Windows programs in the event I need to use them on a computer that isn't mine, since I cannot assume installation privileges.
On it I have the following 'useful' programs:
7Zip, Firefox, Foobar2000, Hexchat, MPC-HC, Skype, TeamViewer, HashCalc, and Sharex
Included are also some niche, less practical programs 'just in case'. CCleaner, a Duplicate Photo Finder (called Awesome Photo Finder), Audacity, GifCam.
I also have a contact.txt file with an email address and my first name in the event I lose the thumb drive and a kind soul wishes to return it.
I'm curious of other people on HN carry around such a thumb drive - and what they have on it. It's always nice to be prepared, and maybe there is something that I may find myself needing that I hadn't thought about. :)
Quick note, I feel like there might be a lot of portable Linux installations and Tails thumb drives.
One is a bootable Ubuntu flash drive.
The second is a cheap USB 2.0 512 Gb flash drive. There I have iso's of tails, ubuntu server, windows XP, and a couple other random distros on one partition. On the other I have personal music and miscellaneous files.