Pidgin - most popular, free, multi-platform IM client that supports most of the IM protocols

Miranda IM - lightweight, resource efficient and easy to use client with powerful plugin support

Ayttm - in case of a service failure, this will automatically fall back to other service protocols of the same person you are chatting to.

Trillian - a Windows application that can connect to multiple IM services, with nice features like Metacontact, Instant lookup, and Emotiblips

meetro - find and chat with people nearby, i.e. location-aware client

IMVU - 3D chat which enables you chat in 3D scenes with your own avatars

Kopete - KDE application, provides users with a single, easy-to-use way to access all of their instant messaging systems

MECA - connect to all of your friends on AIM, MSN, Yahoo! and ICQ from just one list
Centericq - text based multi protocols IM client

Proteus - popular client for Mac OS X that supports multiple protocols

Instan-t - Windows client that allows users to communicate with MSN, AIM, Y!Messenger, ICQ and its IM networks

iChat - IM Client from Apple for Mac OSX

Universal Messenger Plus - a single convenient interface that allows you to easily communicate over different protocols.

Qnext - share your music, photos, and files with anyone, instantly while talking via IM

Aduim - free client for Mac users that can connect to most of the popular messaging services

BitlBee - IRC clients with support of IM protocols
Web based IM

meebo - the most popular third party web based instant messaging serviceeBuddy - chat with your buddies using web and mobile versions

KoolIM - web-based IM that allows you to connect to your email inbox as well as popular IM networks

ILoveIM - another web based IM service with advertisements

IMhaha - supports Yahoo, MSN and AIM but not Google Talk

Goowy - more than just an IM client; offers e-mail, calendar, news, RSS, file storage and more

IMunitive - an alternative to meebo, but doesn’t support Jabber i.e. Google Talk

RadiusIM - web-based IM service with Google Maps integration, which means you can see where your friends are

MessengerFX - unofficial web based IM client for MSN Messenger

ICQ2Go! - web-based installation-free version of ICQ

SNIMMER - chat on MSN, Yahoo, AOL, ICQ and GTalk and make new friends by creating a profile with pictures

IMO - allows you to log in to all chat networks at the same time

Communicationtube - separate login for ICQ, MSN Messenger, IRC and Google Talk contacts

Express Instan-t - simple IM which works with ICQ, MSN and Yahoo
easymessenger - free instant messenger service with built-in RSS support that works with MSN, ICQ, AIM, Yahoo! and Jabber.

mabber - use it on your phone or put it on your website

MSN2Go - Java applet that enables you connect to MSN messenger service

Yafumato Web Messenger - free and open source web-based IM client that allows you to connect to AIM, GTalk, MSN, and Yahoo

MSN Web Messenger - the web-based version of MSN Messenger, without file sharing or video calls

Yahoo! Web Messenger - access to your Yahoo! messenger, chat with your friends and get notified when new email is received.

AIM Express - official web-based instant messenger for AIM

Google Talk via Gmail - chat with your GTalk contacts from Google mail