Google Talk - it's hackable
Google Talk is the thing to mention today so why should I be different? I really hope that it catches on, as it is based on Jabber, which is open, meaning hackable without resorting to reverse engineering and hoping that the protocol does not change overnight as is the case with MSN and ICQ for example.
Presence awareness and IM integration in various contexts can provide some really cool features and is considered a killer feature by IBM in their Workplace product for instance. (A truly crappy piece of software as a whole though)
Finally that Jabber plugin in CruiseControl might come to real use.