Meebo is a really cool website. This website allows you to sign into any of the following instant messengers: MSN, Yahoo, Google Talk, AIM, ICQ, Jabber

This is a really cool website you could use from work or school if they block any of the IM ports.

Also i really liked the user interface that meebo provides.

Check it out: Meebo.com

Revision:

I was using this website yesterday and msn messenger was not sending messages, therefore I can not recommend this site for MSN Messenger. Also gmail already provides Google Talk(jabber protocol) messenger therefore meebo is no use there either.

Here is an alternative, QuickIM, my friend Andrew showed me: http://www.quickim.com/