How? And how can someone see it?
I think nearly all of the wallets includes the feature of adding a message to the transaction. Note that you cannot remove it, it will be permanent to the blockchain and everyone can see it.
-ranochigo
You are mistaken. This is not correct.
None of the wallets provide any user interface for adding messages into the blockchain.
I can include a note in the blockchain.info transaction. I don't get what you mean.
That is not "permanent in the blockchain".
That is a service provided by the blockchain.info company. If you use their wallet, then they let you create a message that they store in their own database and they display on their website. This is not a feature of bitcoin, nor of the actual blockchain. You cannot create those messages with any wallet except blockchain.info