I built a large inventory, product management, CRM, channels management etc etc system. The idea is to integrate all the non-talking and buggy systems needed to run a small e-commerce business (most small e-commerce companies are using money-losing systems and "fixing" the issues with Macro-enabled Excel books). It was also open-source because there is zero way I'd ever take the whole market alone.
The idea, I think, is sound. I worked on the system for over a year, and there are many interesting ideas. The big problem is that it took way more resources to launch than I could ever do alone. I would need sales, support, sysadmin, developers, designers, etc. Everyone calls it my billion-dollar idea, but I truly couldn't and can't do it alone.
The lesson learned is, think big, but not so big you can't handle the work-load. If you are working every day and falling behind two days, the side project is far too large.