Ask HN: Residual income project ideas?

I'm looking for residual income projects related around software. It should be bootstrappable, and done by one person.

The build first approach usually, for example submitting to codecanyon etc. seems to require lot of up front work which I'm open to.

I'm even open towards making a 2d game.

What are some ideas out there floating around? The key point must be that it is something achievable by one person boostrapping.