I've got 2 that are kind of intermingled. Each averages a little over $500/mo on their own though.

Sportsbook API (https://sportsbookapi.com/) - A single API to get odds data from a number of US Sportsbooks

Odds Assist Pro (http://pro.oddsassist.com/) - An odds scanner tool that shows both current odds and things like arbitrage, plus ev, middles. This actually started as just a UI for me to quickly do sanity checks on the API data and eventually grew into a full site. The site is on a subdomain of a site my business partner had built long before we met, so it's kind of positioned as the plus version of that site.

API revenue is really stable and has been pretty consistent slow growth. Pro's revenue is all over the place since it's almost all referrals and promos with big spikes around major sporting events. Probably averages at least $500/mo if you look at the entire year.