Google Talk
Gtalk
gtalk:chat?jid=your account

Skype
Skype
skype:username?call

QQ
QQ
tencent://message/?uin=QQnumber&Site=wangkai.org&Menu=yes

MSN
MSN
- Add a contact to your contact list/添加联系人:
msnim:add?contact=emailaddress@hotmail.com
- Start a conversation with a contact/发起对话:
msnim:chat?contact=emailaddress@hotmail.com
- Send an invitation to start a voice conversation/发送语音对话邀请:
msnim:voice?contact=emailaddress@hotmail.com
- Send an invitation to start a video conversation/发送视频对话邀请:
msnim:video?contact=emailaddress@hotmail.com

*代码中黑体部分替换为自己的帐号即可。
代码使用方法
description or picture
记忆中应该是QQ的最早，然后是skype，05年8月MSN也支持了，后来Gtalk吧。