One of the nicest mail clients on any platform, Sparrow, was just acquired by Google. There's a note on Sparrow's homepage by CEO Dom Leca, which says it all. This is a great move by Google if they allow the team which built Sparrow to focus on making intuitive tools. MacStories got a statement from Google, saying, in part, "[The Sparrow team] will be working on new projects."
For now, Sparrow will remain available but won't be getting any new features. Congratulations and good luck to the Sparrow team -- we can't wait to see what you make next.