Wallets that support 40 byte message attachment via OP_RETURN? Desktop / Mobile? I see that many new Bitcoin 2.0 apps are making use of the additional OP_RETURN parameter for attaching data. I see that max limit is 40 bytes or roughly 40 characters? My question now regarding OP_RETURN is:


So is it one OP_RETURN parameter for each bitcoin transaction or per each input/output address? If its each transaction, so its safe to say its about .0001 btc to send 40 byte message?
Which desktop / mobile wallets support adding a custom OP_RETURN message? (ex. attach a short url link)
Aside from OP_RETURN message attachment, what other methods are there to attach custom data?
So is it one OP_RETURN parameter for each bitcoin transaction or per
  each input/output address?


One OP_RETURN output per transaction regardless of the number of inputs and outputs is standard for 0.9.x bitcoin core clients.


  If its each transaction, so its safe to say its about .0001 btc to
  send 40 byte message?


Yes, but you should not do it without significant reasons.


  Aside from OP_RETURN message attachment, what other methods are there
  to attach custom data?


There are different methods. You can use fake public keys in bare multisig outputs. Or encode some data with values or addresses of other outputs.