USB thumb drives, Mini-SD cards other small digital storage devices have become ubiquitous in our modern world. You probably EDC one, and if you don't, you probably should. You should definitely have one in your go bag. Why? They're just plain handy--not only can you keep important files at-hand, you can carry a slew of portable programs with you for use on public or otherwise unequipped computers. One of those programs should be TrueCrypt, a free and easy to use portable encryption program.
For personal security and survival related purposes, you can encrypt any sensitive documents on your portable drive, or your hard drive for that matter. You don't want scans of your passport, social security card, passport, tax returns, maps of bug out routes or caches, deeds and vehicle titles and other important documents sitting on your drive unsecured. Portable drives can be lost, misplaced, stolen or forgotten, as can laptops. Keeping your files locked down behind some fairly serious encryption is a good way to make sure they don't fall into the wrong hands.
The process is fairly simple--you set up a virtual drive on your hard drive--assigning a file size, encryption level, password and other qualities. This appears on your drive as a file with no extension--basically, if named properly, and not too large in file size, you can disguise this as a junk file. When you want to access the encrypted files or add new files to your encrypted drive, you open up TrueCrypt, "mount" the drive, enter in your password, and then poof, the encrypted drive appears and can be accessed and used like any other portable drive.
The interface is pretty intimidating, but once you've used it a couple times it's no big deal. TrueCrypt has comprehensive instructions and help files, found here.
TrueCrypt is quite powerful--more so than you'd probably ever need to keep a random interloper out of your important personal documents. Few would realize that a random junk file really contained sensitive data. And even someone who knew exactly what they were looking for would have a very difficult time breaking open the encryption--especially without access to code-cracking super computers.
Even in a TEOTWAWKI situation, a portable "survival drive" could be of great use. Having continued computing power after TEOTWAWKI is not entirely unforeseeable, especially as netbooks and ultraportable laptops become more and more prevalent. They require relatively little power and can be powered and recharged even by a fairly small solar setup. You can store an entire library of books, important documents and technical manuals on a small thumb drive--military field manuals, vehicle repair manuals, wilderness survival guides, medical manuals--and any other manner of information that may be hard to find or even completely unavailable after TEOTWAWKI. Good luck carrying all of those in your pack!
One tidbit--Spy-Coins.com sells small, hollowed out fake coins capable of storing a Micro SD memory card. Micro SD maxes out at 16 GB currently, but 32, 64 and 128 GB cards are in the works. Pretty crazy--128 GB of data in something that can be hidden inside a NICKLE! For these SpyCoins, I favor odd foreign denominations--SpyCoins has fake coins in Euros, Swiss francs and British pounds. I like these because you're less likely to accidentally spend one (or have it get mixed up with real pocket change) but they lack the interest factor of half dollar or silver coin. No one will place much value in a Swiss franc coin.