Google Wave

Google Wave looks really cool. It feels like it could be an important leap for communication in general. I recommend you watch some of the video on the link above.
The email part of it seems incredibly powerful, being able to split up your mail content into separate comment trees, effectively forming a tree of discussion points. I know, especially from technical work discussion email, this will be incredibly useful. Everyone currently has a bunch of techniques for doing this now, from inserting comments, using tab indenting and colouring to form such a tree, but it always seems to fall flat and becomes very un-wieldy and difficult to navigate and understand. And coming to such a thread late is nearly impossible. Google Wave will have a "playback" facility so that latecomers can see the the tree of discussion unfold as it did originally. Awesome.
It also has a very powerful instant messaging feature where in this same structure, you can immediately see responses, and indeed see the actual characters appear as typed if desired.
Also importantly, it will be open source, leaving open the door for plugins and custom behaviour, as well as others being able to host their own servers.
The main problem I foresee for Google here is one of scale. It's going to need fast network responsiveness for the IM side of things. Perhaps people being able to run their own servers can help with this, but that sounds like it may be more for corporates and independent user bases.
Waiting with interest for the beta!

[update - other positive reactions here, and here]