After reading ["Have there been attempts at a Bitcoin-powered general purpose database?"](https://bitcoin.stackexchange.com/questions/3968/have-there-been-attempts-at-a-bitcoin-powered-general-purpose-database), I'm wondering if any other uses have been made (or proposed) of the blockchain and Bitcoin protocol that aren't solely about financial transactions (and directly related activities like mining)?

(Perhaps something similar to [Bitmessage](https://en.wikipedia.org/wiki/Bitmessage), but using Bitcoin itself rather than simply taking inspiration from it.)