Google says these. I don't know about any of them

btcd: An alternative full-node implementation written in Go, which does not include a native wallet functionality.

Gocoin: A full-node implementation that has been noted for its dashboard.

libbitcoin: An independent Bitcoin stack, although advertised as pre-alpha.

Bcoin: A JavaScript-based implementation.

bitcoinj: A Java implementation that can operate in a fully verifying mode.