A bug tracking system can be a great project to start junior developers on. It's a fairly simple system that you can use to train them in your coding conventions and so forth. Getting junior developers to build such a system is relatively cheap and they can make their mistakes on something a customer will not see.
If it's junk you can just throw it away but you can give them a feeling of there work already being important to the company if it is used. You can't put a cost on a junior developer being able to experience the full life cycle and all the opportunities for knowledge transfer that such a project will bring.
