Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Since most laptop and desktop computers these days can boot from a USB drive, one of the handiest utilities in my toolbox is an installation of Pendrive Linux.
Pendrive Linux gives you a bootable, fully-working linux system on a USB stick. I use a 2GB ByteStor flash drive personally, with the default install of the Pendrive Linux system. From what I've read, you can go as far as to install larger systems like Ubuntu on a flash drive, but I've stuck to this slightly more compact system (more out of laziness, to be honest).
Portable Operating System
One of the supposed uses of this type of system is portability - you can use it on virtually any hardware, bringing your full linux system and favourite settings with you anywhere.
It's an alluring notion - as someone who jumps between several different systems on a daily basis, having all your settings ready to go is a massive timesaver. However, I've used thumb drive Linux for other purposes...