[zksmk]

https://tildes.net (source: https://gitlab.com/tildes/tildes, github mirror: https://github.com/spectria/tildes, Python back-end, made by dude who worked on old reddit)

https://lobste.rs (source: https://github.com/lobsters/lobsters, Ruby back-end)

https://lemmy.ml (source: https://github.com/LemmyNet/lemmy , Rust back-end, federated, in-development but usable)

https://postmill.xyz (source: https://gitlab.com/postmill/Postmill, PHP back-end)

There's also https://github.com/notabugio/notabug a P2P fork of old reddit UI (that used to be open source itself, old version: https://github.com/reddit-archive/reddit), and there's https://notabug.org/zPlus/freepost, I just found it searching for this stuff, those are the ones I know of.