Free Prolog Compilers
Open Prolog is an Apple Macintosh implementation of Prolog. Open Prolog runs on Apple Macintosh machines running Mac OS 7.5.5 and later, and runs in the Classic environment of Mac OS X.
Visual Prolog 7.4
A free Personal Edition is available of Visual Prolog 7.4 is now available for non-commercial usage only.
The GNU Prolog web site
GNU Prolog accepts Prolog+constraint programs and produces native binaries (like gcc does from a C source)
SWI Prolog is a Prolog compiler, targeting primarily at research and education.
Visual Prolog 7.4 Release
Formerly known as PDC Prolog and Turbo Prolog, Visual Prolog is a strongly typed object-oriented extension of Prolog. There is a Personal Edition for non-commercial personal usage without any time, session, or other limitations
ciao: The Ciao Prolog System
Ciao is a public domain, next generation multi-paradigm programming environment with a unique set of features. See the website for more detail as well as downloadable tutorials and documentation.
P# is a Prolog implementation which translates Prolog to C# source code. This allows interoperation between Prolog and C#, and in particular allows you to combine Prolog back-ends with C# front-ends. P# is based on Prolog Cafe, a tool which translates Prolog to Java.
LPA PROLOG Professional
From this page, you can download a fully-working copy of LPA PROLOG Professional, LPA's first ever Prolog compiler, provided it is used only for personal purposes or study.
B-Prolog is a system for running Prolog and CLP(FD) programs. Like most other systems, it includes an interpreter and provides an interactive interface through which the user can consult, list, compile, load, debug and run programs.
Strawberry Prolog 2.3
There is a limited freeware version, a shareware version and a commercial version of this prolog compiler on this download page.
jProlog home page
Prolog to Java compiler.
Lightweight Prolog-in-Java Interpreter that supports a simplified CUT-less and operatorless subset of ISO Prolog.