BinProlog
Probably the fastest freely available C-emulated Prolog. BinProlog features:
logical and permanent global variables; backtrackable destructive assignment; circular term unification; extended DCGs (now built into the engine as "invisible grammars"); intuitionistic and linear implication based hypothetical reasoning; a Tcl/Tk interface.
Version 3.30 runs on SPARC/Solaris 2.x, SunOS 4.x; DEC Alpha 64-bit version; DEC MIPS; SGI MIPS; 68k - NeXT, Sun-3; IBM RS6000; HP PA-RISC (two variants); Intel 80386, Intel 486/Linux, MS-DOS, Microsoft Windows 3.1 (with DOS-extender go32 v1.10).
Multi-BinProlog is a multi-threaded Linda-style parallel extension to BinProlog for Solaris 2.3.