Another shameless plug? possibly... but apropos.
Great suggestions thus far, so in the spirit of TIMTOWTDI I would suggest having a look at
The Uniqueness of hashes. It's a collection of some techniques for just this sort of operation.
-InjunJoel
"I do not feel obliged to believe that the same God who endowed us with sense, reason and intellect has intended us to forego their use." -Galileo