Ciao - Programming language that builds up from a logic-based simple kernel, and is designed to be extensible and modular.
Scryer Prolog - Modern Prolog implementation written mostly in Rust.
chalk - PROLOG-ish interpreter written in Rust, intended eventually for use in the compiler.
ELPI - Embeddable λProlog Interpreter.
SWI Prolog - Comprehensive free Prolog environment.
pylo - Python front-end for several Prolog engines.
Mycroft - Prolog-like language with compound truth value logic.
Prolog Notes - Some notes taken while working on Prolog.
CLP(FD) - Constraint Logic Programming over Finite Domains.