GNU Prolog is a free Prolog compiler with constraint solving over finite domains developed by Daniel Diaz.
GNU Prolog accepts Prolog+constraint programs and produces native binaries (like gcc does from a C source). The obtained executable is then stand-alone. The size of this executable can be quite small since GNU Prolog can avoid linking the code of most unused built-in predicates. The performances of GNU Prolog are very encouraging (comparable to commercial systems).
Beside the native-code compilation, GNU Prolog offers a classical interactive interpreter (top-level) with a debugger.
The Prolog part conforms to the ISO standard for Prolog with many extensions very useful in practice (global variables, OS interface, sockets,...).
GNU Prolog also includes an efficient constraint solver over Finite Domains (FD). This opens constraint logic programming to the user combining the power of constraint programming with the declarativity of logic programming.