Cool tool for exploring detailed formal mathematics proofs, with heavy hyperlinking. I used to think about building something like this, but less formal, more educational, with some options for abstracting away the details.
I found this while looking to see if they still made Wff 'n Proof games, contemplating getting one for the big kid. Turns out they do. I wonder if it's worth buying $30 worth of wooden cubes and such to do what could be done better with software. But there is an irresistable urge to buy my favorite childhood things for my children.