Off-the-Record (OTR)Messaging
http://www.cypherpunks.ca/otr/

Off-the-Record (OTR) Messaging allows you to have private conversations over instant messaging by providing: a) Encryption – No one else can read your instant messages; b) Authentication – You are assured the correspondent is who you think it is; c) Deniability – The messages you send do not have digital signatures that are checkable by a third party. Anyone can forge messages after a conversation to make them look like they came from you. However, during a conversation, your correspondent is assured the messages he sees are authentic and unmodified; and d) Perfect Forward Secrecy – If you lose control of your private keys, no previous conversation is compromised. This has been added to the tools section of Research Resources Subject Tracer™ Information Blog. This will be added to Privacy Resources Subject Tracer™.