SWI-Prolog is a fast and powerful ISO/Edinburgh-style Prolog compiler with a rich set of built-in predicates. It offers a fast, robust and small environment which enables substantial applications to be developed with it.

SWI-Prolog additionally offers:

* A powerful module system * Garbage collection * Unicode character set handling * Unbounted integer and rational number arithmetic * Multithreading support * A powerful C/C++ interface * GNU Readline interface

XPCE is an object-oriented symbolic programming environment for user interfaces. Although XPCE was designed to be language-independent, it has gained popularity most with Prolog.