One basically puts public domain information online in searchable form and gets massive traffic + ad revenue. Took a while to set up, very boring, but very easy to maintain. Note that being the first mover was very important, the #2 competitor for these niches gets about 20% of the traffic of the winner (and I assume 20% of the revenue).

Another one literally finds links for a particular political topic and sells the combination of links at something like $1000/month. I have no idea why they're so successful-their process consists of Googling said topic for a few hours a week and posting the links on their website-but somehow it works.

Like ThomasLangston mentioned, unsexy markets really work, because you're not competing with the best and the brightest...in fact, you're often not competing with anybody.

Personal anecdote, a family friend sells Usenet access. Seems to be doing very well. Probably the same things that make him successful is the same thing that made my father's business (commercial roofer) so profitable. 1) Unsexy markets. They don't attract competition. 2) Be your own salesperson/secretary/accountant/marketer/janitor/2-3 (cable/roof) monkeys. You don't have to hire bodies if you don't want to and you'll often work harder when it is your bacon on the line.