what do you mean?
There's a thin-client (web-based) version of AIM called AIM express.
There's a couple open-source versions of AIM out there, the most prominent being GAIM.
There's a bunch of third-party apps that connect to AIM as well as other messenging programs (ICQ / MSN / etc), the most prominent being Trillian.
There are apps that strip out all the annoying ads from AIM, such as DeadAIM.
You need to be more specific.