iPhone apps. 2 of 3 of them use the standard free+ads, paid+features model. The other, my most successful, is strictly paid at $1.99.
They were a lot of work up front and some have required minimal maintenance- but supporting them hasn't even taken up even 2% of my coding time in the past year.
I've got a few of these on my back-burner right now... $50-$100 /month, 0 effort.