Ask HN: Share your profitable side project?

It depends on how you define "profit"... I have invested $10 for a domain, and if you exclude my development time, I'm making a profit on UnitPrice.org! [0] It's a website that analyzes specific categories of products on Amazon and calculates the best per-unit price. Lego, diapers, toilet paper, K-cups, etc. Obviously your own time is worth something, but it's fun, and despite barely getting any affiliate fees, it's enough to cover the domain name cost! I also use the website personally, so I've saved money on purchases too.

[0] https://unitprice.org