XSB is a research-oriented, commercial-grade Logic Programming system for Unix and Windows-based platforms. In addition to providing nearly all functionality of ISO-Prolog, XSB includes the additional features, such as evaluation of queries according to the Well-Founded Semantics through full SLG resolution (tabling with negation). XSB's tabling implementation supports incremental tabling, as well as call and answer subsumption.
To understand the implications of SLG resolution, recall that Prolog is based on a depth-first search through trees that are built using program clause resolution (SLD). As such, Prolog is susceptible to getting lost in an infinite branch of a search tree, where it may loop infinitely. SLG evaluation, available in XSB, can correctly evaluate many such logic programs.