Coming from the world of Windows, I found Trillian to be one of the best Collective Instant Messenger which can connect to Yahoo! Pager, MSN Messenger, AIM, ICQ, IRC. The pro version (Pro is the paid version) sport the Rendezvous option where you can have server less messenger within a close intranet. When I shifted to the Apple Mac, my requirement for a universal/collective Instant Messenger was the utmost need and Adium came to my rescue. I never looked back after using Adium.
Adium is a free instant messaging application for Mac OS X that can connect to AIM (ICQ and .Mac), Jabber (including Google Talk), MSN, Yahoo, Bonjour (Compatible with iChat), Gadu-Gadu, Novell Groupwise and Lotus Sametime. Adium X requires Mac OS X 10.2.7 (“Jaguar”) or later; Mac OS X 10.4 (“Tiger”) or Mac OS X 10.3.9 (“Panther”) is recommended. Adium X 0.88 and later are Universal applications which run natively on both PowerPC- and Intel-based Macintosh computers.
For those who want to tweak around and jazz up Adium, it can be done through the Adium Xtras. At the time of writing this article, the current Adium Stable version is 0.89.1 whereas the Beta is at 1.0b5.