I'm looking for a blockchain library: specify how to create transactions, how to collect transactions into blocks, and how to validate blocks, and get an instant working blockchain system to keep a distributed, proof-of-work-powered record of whatever you want. Something like BitcoinJ, but with the Bitcoin-specific parts abstracted out.

Obviously this would be really useful for making altcoins, so I figured this was the place to go hunting. Are any altcoins currently implemented on top of a generic library like this? Or are they all based on independent forks of the Satoshi client?