They say imitation is the sincerest form of flattery. Meebo must be awful flattered right now, because an open-source clone of what the service offered just a year or so ago has come up in the form of Soashable. Sure, it doesn't have the platform, group chat rooms, simple support for Google Talk and ICQ, or the some of the software extensions of the popular Web-based IM service, but if Pidgin is any indication of what can be done in the world of open-sourced IM, Soashable is off to a good start.
Besides the hosted Web app that supports three major IM clients, you can take Soashable and integrate it into your personal site by installing it on your server. The chat windows are currently full of all sorts of debugging gobbledy-goop, but I successfully had conversations with people on AIM and Yahoo without a hitch.
Going forward, this project would make a great plug-in for people with personal blogs or sites. You could create your own versions of the service with skinning and personalization options to match. Meebo has already ventured down this path with sponsorships and advertising campaigns that take advantage of the skinning functionalities, but giving users that power to tweak every aspect of the design to their liking is one of the things that made Trillian so popular with its SkinXML engine.