Instant Messaging Software

The most popular instant messaging systems used today are: MSN Messenger, Yahoo! Messenger, AIM, Google Talk, and ICQ.

Another special IM client, highly praised by the users and techies alike, is Trillian, a fully featured, stand-alone, skinnable chat client that supports AIM, ICQ, MSN, Yahoo Messenger, and IRC.

MSN Messenger
Download here.
Microsoft’s own messenger system that is versatile, pretty and used by millions around the globe. You can even send SMS direct from MSN Messenger to your friends' mobile devices!

Yahoo! Messenger
Download here.
A feature-laden IM system that makes the chatting a real hoot! If you're a Yahoo! user, you'll have access to all the information you store in your Yahoo profile, including your calendar, address book, and customized news.

AIM (AOL Instant Messenger)
Download here.
Also known as: AOL Instant Messenger. You don't need to be an America Online subscriber in order to sign-up to download and use AIM.

ICQ
Download here.
The name ICQ is an initialism on the phrase "I seek you". ICQ was the first internet wide IM client now owned by a subsidiary of AOL.

Google Talk
Download here.
The new kid on the instant messaging block, requires a Gmail username and password. Don't have Gmail? No problem! Get your free account here!

Trillian
Download here.
A one-stop-shop for those who want it all, this IM client is close. Trillian supports AIM, ICQ, MSN, Yahoo! Messenger, and IRC! Both free and paid (Pro) versions available.

Source: about.com