It's foss but got purchased by Kong and started becoming a bit too much of a product for my liking, it has a fork called insomnium that keeps development where it used to be before but it's no longer maintained sadly

Think I used this for a little while. It was pretty much a slightly better Postman, back when Postman itself was less annoying.

Frustratingly, at some point I opened Insomnia and it randomly lost all my requests/history. That soured me on it. Then when I tried it again a couple years later, it had that bad 'freemium' feel, even if it is still FOSS.