Ask HN: What's an added value product a programmer can sell?

I'm starting to think about working on my own on the side of my 9 to 5 job.

I'm also unfortunately acutely aware that the most valuable thing I can offer as a programmer is my workforce, which I have entirely committed to the aforementioned job. This rules out building products for other people.

So I've taken to think, what deliverables can I make that can be sold separately, as complete things? My first thought's always been books: I've written quite a bit about subjects I'm intimately familiar with, but I am reticent to believe those are the only one things we can produce that don't require establishing the kind of service provider relationship normal software development does.

What's more, if such a thing exists, it's pretty possible there's added value variants of it: Think planting grain vs selling cereal bars. If one manages to come up with a deliverable like idk themes or bots or something along those lines, what would an added-value version look like?

Have you guys thought about this? What are your thoughts on the matter?