Save even more bandwidth than BitTorrent by multicasting popular files out from a single server to people who are interested in downloading them. Deal with the reliablity issues and latecomers by using a rateless erasure code so that they can reconstruct the file from just about any sufficiently large piece of the file that they do manage to download. (Sufficiently large will be a few percent larger than the original file.) Of course we'd need IPv6 multicast aware routers to catch on so that this actually works over the open internet.
Ideas: Multicast File Distribution
Idea: a private-key credit/debit card
Prevent identity theft (if you can keep your hands on your card) by using challenge-response authentication. The POS terminal sends your card a challenge, the card encrypts the challenge and sends it back, and the POS terminal checks it using your card's public key (which it fetches from the credit card company). Bonus points: put a key pad on the card, so that your key is protected with a password, and you know your password isn't going into random hostile machines.
So S-cubed also thought of this one. But we thought of it independantly.
And here's all the more reason why!
Smart cards sound like a good idea for this, but maybe a USB device similar to a thumb drive is a better idea. That way we don't need new hardware on our computers to do e-commerce.