Swi-Prolog and the Semantic Web
A long time ago, my first useful experiments with using RDF were based on (after trying other tools) using Swi-Prolog's semantic web libraries. Since then, I have also been using other tools (mostly Sesame, some Jena, and some Franz's commercial AllegroGraph product - which I am planning on writing a short 'applications' book on, BTW, after I finish my Java AI book).
I noticed (see linked PDF paper) this morning that the RDFizing and Interlinking the EuroStat Data Set Effort (riese) architecture (diagram) uses Swi-Prolog on the back end. Very cool. The riese web site itself is interesting: human readable web pages with embedded RDFa for semantic web software agents. (Make sure you view page source on your browser.)