CFAST has been under development since the 1980s. A history of the development is available here. Some of the more recent versions of the software are available for download below.
Version 6.0.10 is still available. This is the version used for NRC validation exercises -- for more information, see the US NRC document on verification and validation of fire models for nuclear power plant applications.)
Version 5.1.1 is still available. This version supports Smokeview and the Advanced Fire Service Interface formats. The source files are also available.
The last DOS version is 3.1.7, and is still available. The source files are also available. The source in the zip file includes graphics calls for on screen plotting. Most developers will want to remove these calls (displayc and below) and compile and run the basic model. For anyone who wants to implement the full model complete with run time graphics, please download the Device.lib distribution available here.
If you need compatibility with Hazard I, version 2.0.1 is appropriate. It can be downloaded here. The source files are also available.