How odd, I was mentioning Dafny in a comment https://news.ycombinator.com/item?id=11930661 just an hour ago, I'm glad they open sourced it Today!

You can try it out at http://rise4fun.com/dafny/Hello

EDIT: They did not open sourced it today, they just moved it from Codeplex to Github today...

http://dafny.codeplex.com/wikipage/history

Interesting, looks like this could even be used as a competitor for Swift (considering the large overlap in features). I'm surprised Microsoft didn't open it up earlier and use it for .NET/Xamarin.

Discussed at https://news.ycombinator.com/item?id=10842714