Ask HN: Website for posting and discuss product idea besides HN?

I don't know if there's a list or not, but many such sites have appeared here on HN over the years. Search through the history here and you should find quite a few. Not sure if they're all still active or not, but it's something that seems to get launched quite frequently.

fwiw here is a "hacker news for x, which lists a lot of such sites, with various purposes: https://hn4hn4x.herokuapp.com/