OTR (Off-the-Record) is protocol that is widely used to encrypt for the exchange of messages in XMPP networks, etc. The difference of this protocol from other encryption systems is that there is no identification of the encrypted message by signature of your interlocutor.
The protocol was designed by Ian Goldberg and Nikita Borisov in 2004.
It received widespread use for the clients of instant messenger due to its ease in use.
It is still updated (at the time of writing, 2015).