1 - Dating sites with targeted communities such as religious peers. 2 - Premium news sites. You see a lot of these with financial sites such as theflyonthewall

1 - Dating sites with targeted communities such as religious peers. 2 - Premium news sites. You see a lot of these with financial sites such as theflyonthewall.com, briefing.com etc. 3 - Quality tutorial sites. lynda.com 4 - I started paying up for an ad free experience and premium features for email.before gmail took off. 5 - Job listings on craigslist. 6 - Membership sites. Too many to list.