I've been reading about "crypto-currencies" like Bitcoin. Here's one based on prime numbers. http://cryptomining-blog.com/tag/prime-numbers/ http://en.wikipedia.org/wiki/Primecoin Many of the existing cryptocoins are based on SHA256 or SCRYPT: http://dustcoin.com/ The largest supercomputer in existence today is the entire might of the Bitcoin mining industry. (Hmmm... I wonder what fraction of those Bluffdale computer cycles are mining bitcoins right now...) The MIT Tidbit crew seems to have replaced "captchas" with coin-miners (as in, "we don't care if you're a bot or not, so long as your computer is willing to mine bitcoins for us"). I wonder if there aren't some other hard problems that would also advance the world instead of simply burning computer cycles. Can't we base a crypto currency on the Riemann hypothesis or perhaps protein folding?