The RepRap software comes in two chunks: the code that runs in the microcontrollers in the RepRap machine itself, and the code that runs on the host (usually a PC running Linux or Windows, or a Mac).
RepRap developers have written generated many alternatives to both chunks, including Skeinforge, a python toolchain for the host, and Marlin firmware written in C++. All the alternatives we know of so far are listed at Builders/Software and firmware and CAM Toolchains and List of Firmware.
The original software used by Darwin was:
- The RepRap software on the host. This is written in Java.
- The RepRap microcontroller firmware. This is written in C.
There is a RepRap CD image that you can download (thanks to Reece Arnott). When you burn that onto a CD it will boot then allow you to run RepRap on any PC.