We used Visual Prolog 5.1 which was developped at the Prolog Development
Center (PDC). The PDC is affliated with Borland International who created
the widely used Turbo Prolog. The IDE and compiler is rather bulky taking
up about 70 Megabytes on one's hard drive. It is, however, the most "complete"
package we could find that was free. It has many leucid examples and
an excellent help facility.
Prolog Development Center: Visual Prolog