Dart is an open-source, scalable programming language, with robust libraries and runtimes, for building web, server, and mobile apps.
Browse pub.dartlang.org for more packages and libraries contributed by the community and the Dart team.
If you want to build Dart yourself, here is a guide to getting the source, preparing your machine to build the SDK, and building.
There are more documents on our wiki.
The easiest way to contribute to Dart is to file issues.
You can also contribute patches, as described in Contributing.