XMPP is a open standarized instant messaging protocol family. It's used widely at services like Google's GTalk, Facebook, StudiVZ and more. Even WhatsApp is using a modified version of XMPP.
transWhat allows you to use any XMPP client to chat with your WhatsApp contacts. XMPP clients are available for nearly every platform: Win, Mac, Linux, Symbian, Blackberry, etc… So you aren't restricted to the proprietary WhatsApp client anymore
If you aren't using a XMPP client, I recommend Pidgin, a multi protocol messenger for Win, Mac and Linuxes.
The source code is available at: http://www.github.com/stv0g/transwhat