Keep Score is a well-established scoreboard style score keeping app on Android with both a paid version and a free version. The free version generates some revenue from ads, and the paid version adds some additional features like auto-texting for scoring updates. It works for nearly all sports in the single app, but it could be easily split up and released as a per-sport app for additional revenue and downloads.
There is a ton of revenue potential if you do some marketing and can get connected in with youth sports leagues. Also, something I wanted to do with this app but didn't have time, is to write some server side code and have live scoring updates for youth sports.