Crowdsourced asteroid discovery (http://www.asterank.com/discover) - only 5 upvotes on HN, but nearly half a million survey images reviewed, with 17,000 potential asteroids marked. Not really working on it anymore.
Free outgoing SMS API (http://textbelt.com/) - not much interest on HN, but about 3M texts sent over the past few years, almost 1000 stars on Github. Requires an hour or two a month for maintenance, responding to issues, etc.
Call Congress (1-884-USA-0234) - single phone number that dials all your representatives one after another. 8 upvotes on HN, but did very well on Reddit and sent over 300 hours of phone calls to Congress in a few days after the Orlando shooting.
Conspiracy theory generator (http://www.verifiedfacts.org/) - Did well on HN with 181 points. About 1M conspiracies generated. On autopilot but still gets organic traffic for ridiculous queries like "snooki illuminati".
Dream logs (http://keepdream.me) - posted 4 years ago but it's a niche tool. 62 subscribers, 2.5k dreams recorded, on autopilot.
Asterank (http://www.asterank.com/) - I submitted parts of this site to Show HN as I added new features. Sold it to Planetary Resources, the asteroid mining company for a small amount.
Meteor showers visualization (http://www.ianww.com/meteor-showers/) - did well on HN, finalist in some Popular Science viz contest.
Dinosaur Pictures database (http://dinosaurpictures.org/) - a few upvotes on HN, about 8k uniques/mo a year later mostly from SEO. This is one of my favorite projects to spend time on.