digsby

5. digsby-The youngest application by far to make
the Hive Five, Digsby has taken the world by storm since we first mentioned it in February. Boasting integration with all of your IM networks as well as your email
and social networking (Facebook and
MySpace included), Digsby is converting
new users left and right with their simple
but appealing formula: IM + Email + Social
Networking = Digsby. Currently a Windows only app, Digsby's developers
promise that Mac and Linux versions are
in the oven—and that they're constantly squashing bugs from the still-young
Windows version.

http://www.digsby.com/

Whatsapp For PC for sure is the best app ever created for instant messaging. There is simply no reason why one won't wanna get it on their phone. 

eBuddy

1. eBuddy – You will just fall in love with eBuddy once you are in. This is the most
powerful and popular IM client for cell phones. eBuddy created the world’s first, independent, web browser-based instant
messaging service as e-Messenger in
2003. This web based messenger
enables you to chat with your MSN,
Yahoo, Google Talk, MySpace, Facebook,
ICQ and AIM buddies. This carry many simple yet interesting features and there
is no downloads, no installation or no
registration required.
Get from here
http://www.ebuddy.com/