IMUnitive is a web based IM service allows connect to MSN, Yahoo and AOL. IMUnitive support SSL, if privacy is needed.

File transfer is also available in IMUnitive. Also conference feature of IMUnitive allows you to chat with your MSN, AIM and Yahoo buddies in the same conference even those buddies don't know each others.

Before you try take a look of their userguide first.

IMUnitive Screenshots: