One day, our wallets will be digital (or maybe our wallets will be replaced by our “smart phones”… I tend to think the former is more likely), but until then, this is a great tip:
Buy a little, thin USB flash drive and keep it in your wallet’s change pocket. This way, since everyone almost always carries their wallet with the, you’ll always have a little bit of storage with you. I keep frequently used software on mine (Firefox, Picasa for the Mac and Windows, Chrome, Synergy, etc) and I use it to move data around.
One thing to watch out for, with this thing being so small, is it’s easy to forget it in someone else’s USB port!
Personally, I carry the Sony Micro Vault in my wallet (but I don’t think I paid $50 for it!), but I think a lot of manufacturers have the same type of flash drive.