ultimate tic tac toe is a nested game, where each grid has a tic-tac-toe board inside of it. the subsquare you choose indexes the supersquare your opponent must go to

ultimate ultimate tic-tac-toe recurses another level. i played this once with a math major friend, it took several days, but i won! (that friend does not talk to me anymore)

wild tic tac toe allows you to place an X or an O. the winner is the one who creates a line of three

misery tic-tac-toe has you win by forcing someone else to cause a line of three

i do not know how to explain quantum tic tac toe