I haven't found a good one yet, but I'd really like it. I sold something to a user on this forum, and I walked him through using the Bitcoin-Qt debug console to generate a 2-of-2 multisigaddress. And then eventually when he got the goods, I tried sending him a transaction to sign and he couldn't get it to sign it. Eventually I walked him through doing a dumpprivkey for his key for the transaction and he sent that to me, and I managed to sign the transaction with both signatures myself. It seems like the underlying technology is there and just needs a nice UI for each side to get and share the public key, generate the multisigaddress and fund it, and both sign it over once both are happy with the transaction. Something that just uses the RPC interface for Bitcoin-Qt may be a good start, but as others have said, it needs to be done right if it's going to be done at all.