Around 2010 or 2011 a friend-of-a-friend introduced me to the card game Projective Set. On Wikipedia, the game is categorized “recreational mathematics” and explained as:
[A] real-time card game derived from the older game Set. The deck contains cards consisting of colored dots; some cards are laid out on the table and players attempt to find “Sets” among them. The word projective comes from the game’s relation to Projective spaces over the finite field with two elements.
If I remember correctly, my quick implementation was the first online version of the game.
The site has been online uninterrupted and without any changes to the original AngularJS 1.2.16 app for a decade and counting, earning it the #1 spot under “External Links” on Wikipedia (as of September 2022).