I used Wave quite a lot, it was great to arranging meet ups of groups of people as you could all have an active conversation and use the "Poll" widget to say if you where coming or not.

I also used it for other things, but organising groups of people was the main use. Once it was discontinued I tried to run the open source version, but it was never really that stable and in the end we swapped back to emails.

It's a great shame to see this dead