comp.lang.prolog is the prolog newsgroup. The comp.lang.prolog FAQ contains information about the various implementations: http://www.logic.at/prolog/faq/
- Ciao-Prolog, which might be noted for it's various extensions (breadth-first search, fuzzy logic, WWW programming interface, concurrency, persistence, and more) http://www.clip.dia.fi.upm.es/Software/Ciao/