One of the interesting use-cases for Bitcoin multi-signature transactions is arbitrated transactions. This allows the buyer/seller to agree ahead of time of an arbitrator that can help resolve disputes and reverse payments, in a way that doesn't put the funds under his full control (unlike an escrow) and does not require him to intervene if the deal ended successfully.

Basically, the buyer deposits his payment to a 2-of-3 multisig address controlled by the buyer, seller and arbitrator. To release the payment, two of the three parties have to cooperate and agree on a transaction that spends the funds from the multisig.

I recently launched Bitrated, that allows to do exactly that. If you're interested, you can learn more in the FAQ page or in the BitcoinMagazine article.