Open-source messaging client Gaim is
about to get a raft of new features, bringing it into the 21
st century. Main Gaim developer Sean Egan has been hired by Google, and part of his job is integrating Google Talk (and thus voice functionality) into Gaim; as part of this, the gaim-vv branch will be merged back into Gaim proper, also giving video/webcam support. Other changes in Gaim 2.0, expected in two months' time or so, will include less broken file transfers and a bunch of new IM protocols, one of which will be "Apple's Bonjour" (by which they presumably mean serverless chat with people on a nearby network).
Instant messaging on Linux is about to get somewhat less sucky (though, with any luck, they'll leave out the full-screen Flash spamming capability MSN is getting). And, with any luck, the changes will end up in Gaim-based clients for other platforms, such as Adium for OSX.
Some of this is already here. Improvements in libgaim from the Google "summer of code" project have already made file transfers with libgaim-based clients way, way better. For example those changes just went into the new version of Adium, and I can now transfer files much more easily than before.