If you look at the site, it says
other clients. They network is based on
Jabber, an already established, open-source, scalable protocol for IM. Apple's iChat AV already supports it, as do GAIM, Trillian, Adium, and half a ton of other IM clients. Send me a message if you are on later and I'll help you set it up.
Everyone else, if you use more than one IM network, or would like to, and you still use AOL's, Yahoo's, or MSN's software, I'd highly recommend trying
GAIM. It will allow you to connect to all of those networks from one program and it does a bunch of things the official clients don't do.