I've never actually had to use this technique, but for years have had this as an insurance policy. Imagine this, you're on a dream assignment abroad. You've got great pictures and your picture editor is desperate for the images. You boot up your laptop and.....nothing; its dead.
I have two USB memory flash drives; one for a PC and one for a Mac. They have a full host of all the software I need to be able to work. Image editing software (Aperture, Photo Mechanic, PhotoShop, FTP programs, settings for FTP and email servers, image rescue and so on).
With most software now purchased over the internet, its very easy just to keep the installer files and their associated serial numbers together in folders. You can also download trail software and not install it, but keep the installer files on your flash key.
If it all goes wrong, I can either pop into an internet cafe and ask them nicely if I can load up my software, find someone friendly with a laptop and do the same, or pop into a store and buy one! On a Mac you could even go one step further and have your whole operating system on a USB flash drive with all your software and settings and if your hard drive goes down, you just boot off the flash drive. I haven't actually tried this yet, and I know it'll be slow, but it will save the day.
If you're wondering which ones to get, Test Freaks have done a round up of 21 flash drives and put them to the test.