imo.im - instant messaging and video chat

imo.im is a web based instant messaging service where you can instant message your friends on AIM, Google Talk, MSN and Yahoo Messenger (similar to meebo, ebuddy, imhaha, koolIM, mabber)
If you sign into more than one account you can link your accounts. When you link your accounts, logging into any one of your linked accounts will automatically sign you into the other accounts.
IMO group chat works across the different networks which means that in a single group chat you can have buddies from AIM, Google Talk, MSN and Yahoo Messenger, all talking to each other.

Mabber - an alternative to ebuddy and meebo

Mabber offers Instant Messaging based on the Jabber/XMPP standard, either web-based and as a mobile application.
Mabber comes with a mobile application that lets you chat and send messages whereever you are. Required is a recent mobile phone equipped with Java.
Similar applications : ebuddy, meebo, koolIM, IMhaha