• analytic tableaux generator: https://www.umsu.de/trees/
• natural deduction proof checker: https://proofs.openlogicproject.org/
• propositional logic calculator (finds models): https://www.inf.unibz.it/~franconi/teaching/propcalc/
• a tutorial on sequent calculus: http://logitext.mit.edu/tutorial
• modal logic playground (for constructing models): https://rkirsling.github.io/modallogic/