Mac OS X: imessage://<your apple id>

e.g.
<a href="imessage://someone@somewhere.com">Send iMessage to a Mac</a>

iOS: sms://<your apple id>

e.g.
<a href="sms://someone@somewhere.com">Send iMessage to iOS</a>

facetime://appleid
facetime://email@address
facetime://phonenumber