- SWI-Prolog
- Stable and free standard Prolog implementation. Targeted primarily at research and education. Windows, Linux and Unix versions available. Comes with a visual debugger and a GUI environment.
- The YAP Prolog System - The University of Porto's Edinburgh-style, WAM-based Prolog engine is available on 32-bit machines running Sun and Linux and 64-bit Alphas running OSF UNIX and Linux. An open-source policy is being implemented with regard to distribution.
- Trinc-Prolog, an object oriented prolog for Windows - Trinc-Prolog is a new development tool for the prolog language. Trinc-Prolog supports the ISO Prolog standard and has extended the prolog language with innovative object oriented features. TPWIN,an OO class library is available to develop Windows applications.
- Visual Prolog - A full-featured programming environment is offered with all the facilities necessary to write mission-critical, commercial-grade applications. A freeware version is available.
