I think I might have mentioned this before but occasionally there are some great sites for sale on Sitepoint. I even bought a new domain there yesterday.
It is one of the few places that people disclose their earnings for their sites. Take a look at this auction, a Christian guitar forum. Not exactly my niche, but it is an excellent example of site that is making a lot of money, with fairly minimal effort from the owner.
I am sure he put a lot of work in it to get it where it is now, but he says he only spends a few hours a week on it. All of the other work is done by volunteers. There are just so many untapped niches out there.