The announcement of the "first developer-oriented version of the Dart SDK" comes on the one-year anniversary of Dart's introduction as a technology preview. The open-source Dart project includes an object-oriented, class-based language, tools, and libraries for building large-scale, structured Web applications. According to the announcement page, the SDK includes:
You can download the various pieces here.
At least some communities must be using Dart for more than just exploratory and learning purposes. The language shows up on RedMonk's annual survey of language popularity, though it's nowhere near the desirable upper-right grid square. To be precise, Dart is two squares down and three to the left of where it wants to be.