Agreed re: writing an ebook. If you have niche content and can package it nicely, you can make $300-$400 monthly. It helps to have some venue for marketing though (blog or other medium).
The hard part is identifying the niche and being patient enough to see it through. A lot of people seem to think they can setup a sales page, put the buy now button on it, and start making $500/mo in 7 days. It doesn't usually work that way.