I want to support in my web server application AOL AIM, MSN Messenger,
and Yahoo! Messenger clients. Where do i find information on the
protocols used by these Instant Messenger clients and would I need to
sign an agreement with the vendors?

Hi felice,

Good day and thanks for your question.

First of all, you need not sign any agreement with the vendors,
provided you respect copyright & trademark laws and do not use any of
the brand names for profit.

For MSN messenger, the following documents give an overview of the
protocol and how to develop such a messenger in VB.

MSN Instant Messenger Protocol
http://www.hypothetic.org/docs/msn/index.php

Official MSN Messenger Service 1.0 Protocol Draft 
http://www.hypothetic.org/docs/msn/ietf_draft.php
The official document from Microsoft describing the initial version of
their protocol. Originial Plaintext Version -
http://www.hypothetic.org/docs/msn/ietf_draft.txt

Venky's MSN Messenger Protocol Documentation
http://www.venkydude.com/articles/msn.htm
A VB developer's guide to writing VB apps utilizing the MSN Messenger
protocol.

jMSN by JALH
http://freshmeat.net/projects/jmsn/
An original open source MSN client written in Perl featuring file
transfer.


For Yahoo messenger,

Yahoo Messenger Protocol (ver 10)
http://www.venkydude.com/articles/yahoo.htm

Yahoo Messenger Protocol v 9
http://libyahoo2.sourceforge.net/ymsg-9.txt

YMSG9 Login Routine
http://www.carbonize.co.uk/vb/ymsg.php


For AOL AIM,

AIM/Oscar Protocol Specification 
http://aimdoc.sourceforge.net/OSCARdoc/

AIM and ICQ use very similar protocols.

I hope this answers your question. If you have any clarifications,
please don't hesitate to ask; I will get back to them as soon as
possible

Thank you for using this service and have a nice day

Warm regards,
aditya2k

Search:
MSN Messenger protocol
Yahoo messenger protocol
ymsg protocol
aim protocol
oscar protocol

The answer seems very appropriate and to the point. Hopefully, the
referenced material will be equally relevant. Thank you, aditya2k.