Libbitcoin btcd Many others over the years but very hard thing to do with decent quality/security for reasons specific to the nature of Bitcoin.

Running v29.1 for now, at least until things calm down or until someone drops a third option, which I hear a couple devs are thinking about working on.

Yes, there is Libbitcoin. It deserves more attention. The main developer is Eric Voskuil (not sure if he is here on nostr)

https://www.lopp.net/bitcoin-information/full-node.html#software Blockcore BTCD Floresta Gocoin Libbitcoin Node nix-bitcoin (hardened security)