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