Google Wave is a new communication service previewed at Google I/O. "A wave is equal parts conversation and document, where people can communicate and work together with richly formatted text, photos, videos, maps, and more." The service seems to combine Gmail and Google Docs into an interesting free-form workspace, where you create a wave and add people to it, that could be used to write documents collaboratively, plan events, play games or discuss a recent news. You can also use "playback" to rewind the wave and see how it evolved.
In addition, Google Wave also includes a context-sensitive spell checker that highlights errors as you type. Google uses the language models built for Google Translate to find words that don't belong in a certain context.
More info: Google Blog