Roine Gustafsson has contributed an open source FPS1 decoder implementation to the FFmpeg project. Note that there are 3 different FPS1 versions and the decoder only supports the first 2. But this is a good start. Check out CVS. And samples are here.
A user on one of the FFmpeg mailing lists brought to our attention a codec called FPS1. The company behind this codec is named Fraps. The application domain for this codec is apparently real-time screen capture of computer game animation, such as in popular first-person shooter games, hence the clever FOURCC FPS1.