The software is open source under the GNU GPL 3 and in a constant process of development.
The full SC3 (with Server and language facilities) is currently available for Mac OS X, Linux and Windows.
As well as developing the kernel, new unit generator plug-ins can be written and many third-party class libraries are available in the SuperCollider language itself.
The main SuperCollider page is:
The easiest way to get hold of SC3 is from a binary download; the above site will give you easy links for current recommended versions for each operating system.
Note that SuperCollider 3.6 and later has a common cross platform look and feel (there is a common SCIDE, and the graphical user interfaces use the Qt GUI library). SuperCollider 3.5 was much more diverse across platforms.
There are some additional sources:
An older page on Linux and SC:
The latest source code of SuperCollider can be obtained using the git version management tool:
So can attempt to build it yourself. This route is not recommended for beginners! The source is available under anonymous checkout from the version control system on github.