The page is a collection of game mechanics/game design ideas. His original goal was to make one mechanic idea a day until he reached 300. He failed his goal, so despite the name at the time of this writing there is only 169 different mechanics.
Still, that’s 169 different game ideas that you can freely use and be inspired by for your next game idea! If you’re stuck in the “I don’t know what to make” rut, click through a few of the entries and see if anything sticks out as particularly interesting to you.
His ideas are public domain and you’re free to use them, just not copy any of his artwork.
I added the link to my own resources page for future reference.
I hope it’s useful to some of you!