VND - Vu Nguyen Learn On The Go

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/) ids can be represented using the xmpp: protocol, e.g.: 
<a class="url" href="xmpp:username@jabberservice.com">IM with username@jammerservice.com</a>

Skype 
Skype can be represented using the skype: protocol. It can be used to open a chat session or make a Skype call. 
<a href="skype:echo-chinese?chat">IM with the Skype echo service (Chinese) </a>
<a href="skype:echo-chinese?call">Skype call to Skype echo service (Chinese) </a>  

ICQ 
ICQ does not have a URL scheme. Rather ICQ links on the web use HTTP hyperlinks to download a resource of content-type application/x-icq which then the operating system opens with the user's ICQ client. 
Thus we can markup those HTTP hyperlinks with the content-type set explicitly to communicate the ICQ semantic: 
<a class="url"
   type="application/x-icq" 
   href="http://www.icq.com/people/cmd.php?uin=[ICQNUMBER]&action=message">
 Contact with ICQ</a>

Replacing [ICQNUMBER] with the user's actual ICQ number.