I just watched this
video preview for Google Wave.
I must say I am highly impressed. While I don't think this will necessarily replace e-mail, I think it could easily gain widespread use as an alternative to both e-mail and social networking applications (like instant messaging) and websites (like message boards, MySpace and FaceBook).
This could also be a powerful tool for playing tabletop RPGs online. All you would need to add is a dice roller and some basic image editing similar to what you can do in Powerpoint. Then the GM can upload gridded maps and everyone could move icons representing characters and monsters around on the map.
I think there are already good dice roller apps out there which you can imbed in other media, and the image modification is a feature that may well get added to Google Wave to allow group editing of visual media similar to the group editing of text that Wave alrerady allows.