Apple has launched Swift , currently available at 2.1. It is available at its own web location with instructions on getting set up on multiple platforms, focusing mostly on Mac OS X and Linux platforms. The source code is available on github under the Apache 2.0 license. The API for the GUI is Mac only, but the Linux version can work with GlibC without any GUI at this point.
On Microsoft Windows there is hope for adding swift bindings to Cocotron a cross platform toolkit which works with Objective-C API similar to Apple’s. All source for Cocotron is under the is under the MIT license.
On Github, there is a programming lanugage feature page which lists Swift as being developed in C++.