This is something that has been in my mind for some time: If I understand correctly, the bitcoin generating process is based in computing SHA's.
So, I wonder if those computations could be used for something else? Maybe having a database similar to Rainbow Tables, which allows people to look for previously calculated SHA's to avoid calculating them...
Almost any proof-of-work algorithm could be used. However, if you add an algorithm which is useful to the outside, it decreases the cost of attack (since you earn something probabilistically on top of simply a block reward). Depending on what kind of algorithms you use, this could be dangerous for network security.
Haha, aside from the obvious problems of storing 2^256 hashes somewhere, having a database system that can handle all of the btc client connections, and undermining the actual proof of work system behind bitcoin's credibility, you'd be creating a useless rainbow table that didn't have any common cleartext passwords hashed in it.
So, I wonder if those computations could be used for something else? Maybe having a database similar to Rainbow Tables, which allows people to look for previously calculated SHA's to avoid calculating them...