Simple web "live chat" software (LAMP stack) that integrates with Jabber/Aim [closed]
(Disclaimer: I work for Jabber, Inc., the commercial company behind the product I'm about to pimp.)
(source: jabber.com)
The JabberWerx AJAX libraries do exactly what you want. You include a reference to a Javascript library, add a div tag where you want the chat to go, and add a couple lines of configuration javascript to hook the two together. There's also a one-to-one mode. User accounts can be created on the fly if you like, as well.
Sorry for the ad, but I think it's exactly what you want.
This wouldn't be that hard, if you implement the Oscar protocol that AIM uses. It's not very complex, and that would allow you to build a nice web based AIM client for your website. There may be a 3rd party solution that you could use, but as far as I know, Oscar is pretty trivial.