I put ads on http://jsonblob.com about a month ago and didn't really expect much, but have been pleasantly surprised! I should have put them on it when I launched it a little over a year ago...

In terms of breaking even, it's in the black if you don't consider the time I spent building it (and rebuilding it when Grails started falling over). It runs on a single Heroku dyno, so that's not casting me anything and uses the free tier of MongoHQ.