Google Wave, a strange product that noone REALLY knew what to use it for. In my opinion, it was just too different. As of now, Wave is no more:

Google is waving goodbye to Wave, a product which the company said would transform the way people communicate online.

It blamed poor take-up by users of the service, which was launched to great fanfare just one year ago.

At the time Google described the tool as "how e-mail would look if it were invented today". Source
I don't think any of you use it regularly, but you'll be able to continue using it until the end of the year. Now, wave goodbye everyone .