Domainr -- http://domai.nr/ -- a search engine we built for finding available domain names.

~80% of its revenue is registrar commissions from successful referrals (domain name purchases, hosting, ssl certs, etc.), and the other 20% is from ads (from adpacks.com).

We launched it in 2008, and both traffic and revenue have been growing slowly but steadily ever since. These days it's covering our respective apt rents (in the Bay Area) each month.