I have a few revenue-generating side projects, most notably StatusGator[1] which monitors status pages.
But the problem I always have with selling side projects is that they are worth a lot more to me in emotional value than they are actually worth monetarily. I've put a lot of heart and soul into StatusGator, not to mention a lot of time, and selling it for what it's worth based on monthly revenue ($1k/month) wouldn't nearly cover that emotional value. And so I hang on to it.