I have a quite simple web site with some calculators for taxes and stuff, that I originally built in 2007. The Google AdSense and affiliate income has grown from about $1000 per year to almost $1000 per month.

It is "passive" in the sense that I respond to the occasional e-mail (once a month), update the data once a year, and add another calculator when I feel like it.

A few years back, I was in the same position with another (online casual gaming) website, that I sold for 2.5x the yearly revenue. Looking back, I should probably have kept that site as well.

Pro tip: quality content beats SEO in the long run. Be the tortoise.