Matt Thomlinson posted some interesting ideas.

Other possible net services: random number services, For crypto use that's not very helpful - if the numbers go across the net, other people can see them. But they may be somewhat helpful as seed material for your own random number generator, along with hashes of your memory, random Ethernet traffic, etc.

A slightly different application is for tax purposes - you may have a machine that needs to be used for business over 50% to make it a business asset rather than a personal one, and the amount of depreciation you can deduct is proportional to the business usage. Even if you can't make a *lot* of money selling random numbers and big primes or attempting to crack other people's keys for a small fee, it *can* be business, and certainly should keep the CPU depreciating rapidly at night....

Also, one could set up a hashing/signing service, more akin to a time stamp service. [....] "Digital Notary" system?

There's certainly a need for such services. You have to be careful to avoid stepping on Bellcore's work, since I think it's patented, but related services may be practical and profitable. You have to decide how much you're willing to trust the timestamp that the service generated, as you would for a human notary. With a digital notary, there's a risk someone could hose the clock on the notary's machine, get something notarized, and reset the clock, so even if the notary's being perfectly honest it's not risk-free.