I did some testing of eMessenger's web-based IM and have finally found a solution that (for me) works behind the corporate firewall and lets me connect to MSN. It's not as slick looking as MSN Web Messenger, but this one (for me) works!
MSN has their own version of a web messenger now - http://webmessenger.msn.com - not sure if it works behind firewalls or not, but it's a bit nicer than eMessenger. As can be expected, it works best in IE6.
