Web Design
AOL Instant Messenger (AIM) AOL Instant Messenger (AIM) ids can be represented using the aim: protocol. Many who publish their AIM ids do so with clickable URLs e.g.: <a href="aim:goim?screenname=ShoppingBuddy">IM with the AIM ShoppingBuddy</a>
Yahoo Messenger
Similarly, Yahoo Instant Messenger (YIM) ids can be represented using the ymsgr: protocol. And similarly many publish their YIM ids as clickable URLs e.g.: <a href="ymsgr:sendIM?SomeYahooFriend">IM with SomeYahooFriend</a>
MSN Messenger
MSN Messenger (MSNIM) ids can be represented using the msnim: protocol. And similarly many publish their MSNIM ids as clickable URLs e.g.: <a href="msnim:chat?contact=joebob@hotmail.com">IM with joebob@hotmail.com</a>
XMPP (Jabber)
Extensible Messaging and Presence Protocol (XMPP) (http://www.xmpp.org/)...