Multiple IM Standards, One Client

[Gorilla]

If you're like myself and communicate with various people who use different IM services, you may want a single client with a uniform interface that handles some of the most popular services.

Pidgin

Pidgin is an instant messaging program for Windows, Linux, BSD, and other Unixes. You can talk to your friends using AIM, ICQ, Jabber/XMPP, MSN Messenger, Yahoo!, Bonjour, Gadu-Gadu, IRC, Novell GroupWise Messenger, QQ, Lotus Sametime, SILC, SIMPLE, MySpaceIM, and Zephyr.

Pidgin also has quite a few handy plug-ins that offer a host of useful features. One of them worth mentioning is an encryption plug-in (Pidgin-Encryption).

Available Here: Home | Pidgin

A console client is also available at the pidgin website named Finch.

Adium (MacOS X)

Adium is a free instant messaging application for Mac OS X, released under the GNU GPL and developed by the Adium team. With Adium, you can connect to any number of messaging accounts on any combination of supported messaging services and then chat with other people using those services.
Supported services

* AOL Instant Messenger (Including ICQ and .Mac)
* Jabber (Including Google Talk and LiveJournal)
* MSN Messenger
* Yahoo! Messenger (Including Yahoo! Japan)
* Bonjour (Compatible with iChat)
* MySpace IM
* Gadu-Gadu
* Novell Groupwise
* QQ
* Lotus Sametime

Available Here: Adium - Download

Both of these clients are completely free and open source.

[Gorilla]

A much better encryption plug-in for securing Instant Messaging is the Off The Record plug-in. It ships with Adium and is available for download and use with pidgin.

Off-the-Record (OTR) Messaging 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.

It definitely beats the plaintext form IMs normally fly around in.

Website: Off-the-Record Messaging