Hmm, i guess this really depends on your current skillset.

If you are considered an expert in a niche enterprise software, you could provide some form of advisory or consulting services.

Alternatively, i got a couple of mates who write mobile applications as residual income, but to be honest its not a lot of money but it does help to build up their portfolio.

Hi @johnny - as well as these suggestions, how about seeking out other lone engineers (I suspect there are plenty of them) to exchange ideas and perhaps come up with something cool and sellable ?

There are many ways to make money as a developer. Here are 4 examples:

1. ThemeForest - create themes in bootstrap, or for Wordpress and sell them here
2. Envato - write tutorials and get paid
3. oDesk, UpWork etc. - find a side-gig and work it
4. Write an app

To make up some residual income, I picked up writing(scotch.io, have some drafts for SitePoint). Also, I recently started making themes for ThemeForest and scripts for code canyon.

Freelancing is also another way to go, you could try out PeoplePerHour or Upwork.