Meebo.com is a web based IM that lets you log into your IM networks (e.g. GoogleTalk, Yahoo!, MSN, AIM, ICQ, Jabber) from any computer with a browser and internet connection with no firewall issues. It employs technologies (like AJAX) to make it act more like a desktop application, which dramatically improves usability. Many users enjoy the benefits of a native DHTML application, as it does not consume as many system resources as a Java applet (such as ICQ2Go or Yahoo Web Messenger).
Moreover u can create a meebo ID, so just type ur user-ID & password once n log into all ur fav messenger.
So if at office your are not allowed to use messenger, then try this web based one.
For publishers, there is a thing called WIDGET that u can directly paste on to ur site code so u can talk to ur site visitors directly!!! U can see live example of this... jus check-out my sites lower-right corner!
As meebo is web-based it is 100% platform independent!!! At the time of writing this, it does not support following browsers:
IE 7.0 Beta & Beta 2 and Opera 8.53