Miranda IM

Miranda IM is a multi-protocol instant messaging client for Windows. Very light on system resources and extremely fast, Miranda IM requires no installation and can be made to fit on a single floppy disk or USB drive.
Featuring a powerful plugin-based framework and boasting over 350 plugins, Miranda IM is one of the most flexible and customisable messaging clients on the planet. To see how some of our users have customised Miranda IM, check out the screenshot section.
Protocol Support

• AIM (AOL Instant Messenger)
• Facebook (See instructions)
• Gadu-Gadu
• IAX (Inter-Asterisk Exchange)
• ICQ
• IRC (Internet Relay Chat)
• Jabber
• MSN
• Netsend
• Tlen
• Yahoo
• And more…

Supported Operating Systems
Windows 95 / 98 / ME / NT / 2000 / 2003 / XP / Vista / 7
(Download) (Source)
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 SecureIM (MirandaIM Plugin)

Plugin for encrypting your messages on AES192 in legacy mode, RSA4096 for session keys transmission, AES256 for sessions keys in new mode or PGP/GPG. Support Unicode, tabSRMM and CList Advanced icon.
Require Crypto++ dll, download from http://addons.miranda-im.org/details.php?action=viewfile&id=2669
Source code available at svn http://secureimplugin.googlecode.com/svn/trunk/
To decode RTF messages you need rtfconv.dll library, you can get it from http://faith.mirandaim.ru/archive/SecureIM/
Forum thread http://forums.miranda-im.org/showthread.php?t=4768
(Download) (Source)