This all assumes you are on 10.4 (as that's what I'm running).
Ruby is already installed by default (which is nice). You can confirm this by opening the Terminal and typing:
All being well you should get
If you don't get this back then try installing the developer tools as ruby might just be installed with them.
Download the source rar file (and if your Safari is like mine remove the .txt extension it adds). Use Stufit to extract the code.
Then try and run the code. At this point I cannot get going. I don't know much about ruby but it seems to be trying to load a shared library (digest/crc32_in_c) included in the "source" and can't, probably because the library is compiled for Windows/x86. Source should be just that, the source code, not source code and binary files mixed.
Anyone else got any ideas?