If e-mail was invented today, Google Wave is what it would look like. It combines the best of e-mail, instant messaging and online collaboration into one tool.
I've used it to track information for a major project and collaborate/share with other people involved with the project. It's great for collaborating on something as simple as a grocery list or as complex as planning a vacation.
Google Wave is vastly superior to ordinary e-mail and, if marketed and developed correctly, could eventually become more widely used than e-mail. It is my hope that it does, because the current e-mail system is based on a protocol developed in 1982, before there was Spam. It's a very trusting protocol, which is why fighting Spam is such a massive effort. Google Wave fixes that problem and many more.
Right now it's in beta and not open to the general public. However, I have some invitations left if you're interested in getting a Google Wave account. Just let me know.