There is a list of bounties on the wiki here:
How are the BTC pledges collated and eventually paid? I assume there is some kind of escrow system but I can't find out anything about it.
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Multisignature transactions can act as escrow.
A transaction could require a party and a trusted third party to complete, or it could be the trusted third party alone who signs to complete the transaction.
Usually they're a multisig transaction between a couple of core developers. One recent one was a 2-of-3 between Greg Maxwell and a couple of others. It's mainly just to prevent hit-by-a-bus scenarios, we trust in these people absolutely.