There is a service that uses bitcoin blockchain to prove that certain data exists at a certain moment of time [link removed] . This is done by generating a bitcoin transaction (unspendable) to two specially crafted addresses.
This is a very inefficient and destructive way to perform this tasks. I would not recommend using or promoting this service.
I wrote a system that does the same thing, but without spamming the blockchain with unspendable junk.
I never released it. If there is some demand for such a service, I can finish it up and put it online.