Prooftoys is a visual proof assistant and the engine behind Mathtoys (http://mathtoys.org). The Prooftoys implementation is based on Alonzo Church's simple type theory as formulated by Carnegie Mellon University Professor Peter Andrews under the name Q0. Simple type theory is suitable for construction of most of mathematics, comparable to first-order logic plus set theory. It uses a minimum of simple, understandable concepts, expressing them with a handful of axioms and inference rules.
Prooftoys is written in TypeScript, which runs as JavaScript in the user's web browser. It is designed to be highly interactive and delivered to users through the Web, so they can play with it instantly. Prooftoys has a web site with pages for interacting with it at http://prooftoys.org/.
Files for generating the website are at http://github.com/crisperdue/prooftoys-site/.
Mathtoys is an interface to Prooftoys tuned for working high school algebra problems. Its unique HTML and code is also on GitHub at http://github.com/crisperdue/mathtoys.
The Prooftoys engine, together with its user interface, are designed to embedded in other websites. Contact the author for details.