By "Bitcoin Core" alternatives I mean full validating clients that follow consensus rules (do not fork the blockchain... because that would be an altcoin).
The ones I know are:
Bitcoin Knots: This is developed by Luke Dash Jr and has some experimental features that sometimes are added into the Core client:
https://github.com/bitcoinknots/bitcoinLibbitcoin: I think this one was founded by Amir Taaki:
https://github.com/libbitcoin/libbitcoinBTCD: This one doesn't have a wallet and is written in Go:
https://github.com/btcsuite/btcdBitcoinJ: Very old client from back in the day, I think it was started by Mike Hearn: https://github.com/bitcoinj/bitcoinj (removed because not a full validating node)
Bitcore: Looks like Trezor uses this one, but also Bloq...:
https://bitcore.io/Bcoin: Some exchanges use this one like Bitwala:
http://bcoin.io/Im sure there are more.. post yours. I think these efforts are good and positive for Bitcoin overall. As long as they follow consensus rules, it's good that several different teams are working on different clients.
Your list there covered pretty much all the ones I could think of off the top of my head.
But there's also another one if I remember correctly, which was Coinbase originally using their own implementation written in Ruby. Now this was a pretty long time ago, so it could be discontinued, but I do remember it existing. I'm not sure if it had a specific name or anything though either.