meebo.com
While going through popular links in del.icio.us, I found a great link http://meebo.com. It is a great online messenger service. It contains AIM, Yahoo! Messenger, ICQ and MSN. For long time, I was in search of such service. This site has fulfilled it.
update: September 19, 2005, Meebo also added Jabber or GTalk support. Now more powerful.