Cycles can be used as a standalone renderer, but it is integrated into applications as well.
Cycles is fully integrated into Blender, a 3D Open Source content creation suite. Blender can be downloaded from blender.org.
Cycles has been integrated into Poser under the name “Superfly”. You can get Poser from smithmicro.com.
Integration is in progress and should be available with the upcoming Rhino 6.
If you want to take a look at Cycles source code or contribute code, please visit developer.blender.org.