OTR

Instant messaging typically goes through the server of the protocol you're using, for example, Microsoft in the case of MSN, and anyone else between you and your recipient can read all of your instant messenger conversations as it's all plain text. In comes Off-the-record messaging which adds encryption to the mix, so this is no longer an issue. With OTR (taken from the OTR website):

1. No one else can read your instant messages.
2. You are assured the correspondent is who you think it is.
3. 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.
4. Perfect forward secrecy
5. If you lose control of your private keys, no previous conversation is compromised.

Pidgin is an IM client that does all of the popular IM protocols, this includes Yahoo!, MSN, Jabber, AOL, IRC, Sametime, etc.

GNU/Linux, or BSD

Download Pidgin

If you use windows, use the first windows installer, don't choose the one with no-gtk or debug in the name. Also if you use windows remember if the version you're downloading starts with a 1 or a 2.

Download OTR for pidgin

If you use windows, download the appropriate version.

MacOS X

Adium X

Windows

Pidgin

Download OTR under "OTR plugin for Pidgin" choose the latest Windows version