GNU Prolog is one of my favorite Prolog compilers. True, it’s not the fastest system nor the one with most features. It’s, however, one of the free and open source Prolog compilers that more closely complies with the ISO Prolog Part 1 standard. GNU Prolog is one of the compilers I use daily to develop and check the portability of the Logtalk code base. Daniel Diaz just released version 1.3.1:
Support for this new version is already included in the latest Logtalk development version (r4695).
Prolog development tip: whenever you need to check the ISO Prolog standard and you don’t a have a copy at hand, take a look at the GNU Prolog manual. ISO Prolog predicates are clearly marked and described in detail (including template, modes, and exception terms).