"WI-Prolog offers a comprehensive free Prolog environment. Since its start in 1987, SWI-Prolog development has been driven by the needs of real world applications. SWI-Prolog is widely used in research and education as well as commercial applications. Join over a million users who have downloaded SWI-Prolog."
The (semantic) web is one of the most promising application areas for SWI-Prolog. Prolog handles the semantic web RDF model naturally, where RDF provides a stable model for knowledge representation with shared semantics. It turns out that Prolog is also quite capable of providing web-services (HTTP), especially where it comes to dynamic generation of HTML pages and providing data for Javascript web-applications using the JSON serialisation.
"ClioPatria is a SWI-Prolog application that integrates SWI-Prolog's the SWI-Prolog libraries for RDF and HTTP services into a ready to use (semantic) web server. The core server has an integrated package manager that allows distributing and using add-on libraries: CPACK."
Flora-2 is an advanced object-oriented knowledge representation and reasoning system. It is a dialect of F-logic with numerous extensions, including meta-programming in the style of HiLog, logical updates in the style of Transaction Logic, and defeasible reasoning. Applications include intelligent agents, Semantic Web, knowledge-bases networking, ontology management, integration of information, security policy analysis, and more.