Google Wave is a sort of combo e-mail/instant messaging/collaborative tool.
There's a 1 hour 20 min video of it on the Google Wave site. I encourage
everyone to watch it. It demonstrates so many complex, insane engineering
problems and bundles them into one cohesive workflow. It was developed in
secret for two years by the same team that came up with Google Maps.
I think given the right marketing and packaging, this could absolutely
revolutionize instant messaging, e-mail, collaboration in the enterprise,
and feed-based activities (bloggish/twitterish things).