Sparrow actually had plans to launch a version for Windows OS!
Panos Tsimpoglou is the founder and software architect of Anamo.eu.
He has an interesting blog post app on how he was coding a Windows port of Sparrow for the company.
It was expected to see a public release sometime next month. Though the Google deal has ensured that it would never come out.
Tsimpoglou has indicated that he might open source the code. Here is what he said:
Well I guess it’s a Beta app of a dead brand. Who knows? Maybe OpenSource it. Rebranding it seems such a big task and I am truly tired of making great apps for.. me. If nothing happens and it stays with me, I may try to gut it, simplify it(because yes, Sparrow is bloated, trust me, french-bloat), re-imagine what Email could be(unread? so passe), and put it out for my (and maybe your) pleasure.