Personal Project Ideas

I am looking for a new project to start on at home in my spare time, something that keeps me challenged and using new technologies (definitely AJAX, possibly even Ruby on Rails).  The latest projects I have worked on setting up are background procedures for all of our personal computer data onto a network storage device.  I then periodically swap out the hard drive in the device with a new one and store the hard drives in a fire proof safe.

Anyways, I want something a bit bigger, something that may take a while, but can have easily identifiable milestones to show progress.  Any ideas would be helpful, I have tossed around the idea of starting a new E-commerce Project, one more surrounded by a different architecture than I used for cpCommerce, but just as modular.