What are good side projects? I get this "cool stuff that I don't get to do at work" idea, but I think it's vitally important to have side projects that have a different gratification profile than one's main work. If you work as a developer of any kind, most work will be on massive projects spanning at least months, often years. For a side project to be fun, I think its important to have a different pace. For example trying out a new technology and doing some cool hack quickly, in a few hours, rather than "doing it right" slowly and meticulously. Maybe side projects should be mainly about learning or trying out things rather than doing things, exploration not exploitation of existing skills.
A good side project is anything you think it is. The most important aspect is that it must keep you engaged, whereas a project at work has to be profitable as its primary aspect.
The "gratification profile" should be what works best for you and your personality and what you need to accomplish. If your side project is building a boat to cross Lake Superior, then it's fine if it takes years to complete. If it's automating your garage door to open when your car arrives, it might be done in a weekend.
The point of the project is that it should be about you. What you want to do, on your terms, at your pace. Otherwise, you might as well just get a second job.
For me its automating turn-based role-playing game mechanics using .NET technologies. Turned out to be a much longer term project than anything I've worked on "for hire".
