I run https://www.goread.io/ which is a subscription-based RSS reader. It's open source and runs on google app engine. It costs ~$2k per year to run and I make between $2k and $3k profit per year for nearly zero effort. Stripe handles payments and app engine keeps the servers running.

I consider it a high success because for the amount of marketing I've done in the last 2 years (exactly zero), it still has paying users. If I did a few months of code improvements and marketing pushes I could probably increase that income a few times.