Set Up a Wiki
"Whenever a video game is announced that I have an interest in, I will register a domain to make a wiki for the site.
"I will actually make the site, but last year I was able to sell skyrimwiki.com for $1,000. My success story is dredmorwiki.com: pretty much the de facto information site for Dungeons of Dredmor. I only make about $50 per month from advertising from that site, though.
"Setting up a wiki is pretty easy, and users actually generate the content themselves, so there's not too much maintenance involved.
"The trick is right place, right time. There's a lot of luck involved, but it's a fun hobby."