Membership sites have proven to be the most profitable if you have the abilities to get traffic to your website. Not that I really want to mention it, but the perfect example of a very profitable membership site would be an adult site. The site itself offers only teasers and previews of what's inside and people need to pay a monthly fee to see your whole site. The big money starts to really roll in because as your site ages you get more and more members and typically membership sites sell monthly for around $10 to $25 per money. Image if you had 200 members. Although, most webmasters consider this the best profitable site, It also has it's demands. The site must be designed very well and have a lot to offer in the member area. If not you surely will get bad publicity from people who have been members. No one likes to get ripped off. Not only that, but your site will need an active security system, support ticket system, and bank system to be able to take money or paypal.

Of course adult sites are not the only way to go. Actually you can have a membership site built from anything almost.

Here's a list of some of the millions of niche's you could have a site about Link selling site, site selling, online auction, autotrader type site, etc. Basically anything that is in high demand that people are willing to pay for.