We created a list of more than 140 tools that can be used to create tutoring systems, from complete tutoring systems to low-level tools for preparing instructional materials. Based on this list, we present a preliminary ontology of system dimensions that can serve as a base for a comprehensive review or in building systems. We also note that: (a) such surveys of tutoring systems will always be incorrect because they are incomplete, (b) such reviews are not worth pursing indefinitely (because they will never be complete), and, however, (c) comprehensive reviews, or a "living platform" like a wiki, may be immensely useful for stakeholders. The results provide several insights: (a) the scope of tutoring systems includes a great range of dissimilar tutoring architectures, as well as an even wider range of supplementary tools; (b) systems vary on many dimensions (we provide an initial set); and (c) no tutoring system is likely to gain widespread adoption for all use cases because of the variety of learner and other stakeholder needs. This suggests that a wide variety of tutoring architectures are needed. The ontology and dimensions may be helpful in development, funding, and acquisition decisions.