Microsoft just released public beta of MSN Web Messenger. I looks really cool and it should work with IE and in Netscape/Mozzila. Haven't tested though.
What is MSN Web Messenger?
MSN Web Messenger lets you talk online and in real-time with friends and family using just a web browser! Use it on any shared computer - at school, at work, at a friend's house or anywhere you can't install the MSN Messenger software.