user123: There is a list of bounties on the wiki here:
https://en.bitcoin.it/wiki/Active_Bounties
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.

user-1: 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.

user13413: 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. 

123: <p>There is a list of bounties on the wiki here:</p>

<p><a href="https://en.bitcoin.it/wiki/Active_Bounties" rel="nofollow">https://en.bitcoin.it/wiki/Active_Bounties</a></p>

<p>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.</p>
 -1: <p>Multisignature transactions can act as escrow.</p>

<p>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.</p>
 13413: <p>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. </p>
