You can use Yahoo and MSN with iChat, but it requires using a XMPP (formerly jabber) gateway.  MacAppStorm has a great write up on how to configure using PSI.  The only additional caveat that I can add is that I ended up using a different gateway than the one suggested in their write-up.  xmpp.ws was down when I was testing this out, so I chose jabber.iitsp.com