I currently run a web hosting business and two directories and have a few more businesses in the pipe works
I run membership/affiliate programs. You get a list of subscribers, affiliates, and monthly income. On top of that your army of affiliates with the right incentive in place promote your site for you. So you get traffic and income. Best of all you can build one about any niche and use any product to build around it.
I run a web hosting and web development company as well as PHPGator.com which sales a couple of PHP Scripts (soon to be 3).