OTR

Off-the-Record Messaging, commonly referred to as OTR, is a cryptographic protocol that provides strong encryption for instant messaging conversations. OTR uses a combination of the AES symmetric-key algorithm, the Diffie–Hellman key exchange, and the SHA-1 hash function. In addition to authentication and encryption, OTR provides perfect forward secrecy and malleable encryption.

Strengths
OTR allows you to have private conversations over instant messaging by providing:
 * Encryption
 * No one else can read your instant messages.
 * Authentication
 * You are assured the correspondent is who you think it is.
 * 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.
 * Perfect forward secrecy
 * If you lose control of your private keys, no previous conversation is compromised.

Native

 * Adium (Mac OS X)
 * BitlBee (Cross-platform)
 * Blink Cocoa(Cross-platform)
 * CenterIM (Unix-like)
 * climm - dead (Unix-like)
 * ChatSecure (formerly Gibberbot) (iPhone)
 * Jitsi (formerly SIP Communicator) (Cross-platform)
 * MCabber (Unix-like)
 * Phoenix Viewer, a Second Life client (Cross-platform)
 * Profanity: OTR page
 * Psi/Psi+ (Cross-platform)
 * Xabber (Android)
 * yaxim (Android)
 * Zom (Android, iPhone)

Plugin

 * Gajim (Cross-platform) with Off-The-Record Encryption for Gajim
 * irssi (Cross-platform) with irssi-otr
 * Kopete (Unix-like) with Kopete Off-the-Record plugin
 * Miranda IM (Microsoft Windows) with MirOTR - OTR for Miranda IM
 * Miranda NG (Microsoft Windows) with MirOTR - OTR for Miranda NG
 * Pidgin (formely Gaim) (Cross-platform) with OTR plugin for Pidgin
 * Poezio with pure-python-otr
 * Salut à Toi with plugin_sec_otr.py (installed by default)
 * Trillian (Microsoft Windows) with Trillian OTR
 * WeeChat (Cross-platform) with weechat-otr
 * XChat (Cross-platform) with xchat-otr