Baseball:
Phantom tag - If you’re stealing a base or trying to score, and the ball beats you there, the umpire will call you out in many cases. This is true even if the fielder or catcher misses the tag before you touch the base.
Catching line drives - If there is a runner on 1st, and a line drive is hit to the 2nd baseman (for example), he won’t intentionally drop it to secure an easy double play. But of course, baseball did need to “write” the infield fly rule, so sometimes you cannot trust players to respect the unwritten rules.
Ball 4 - With 3 balls and no strikes, a pitch has to be pretty far out of the strike zone to be called a ball.
Tennis:
Net Cord - If you hit a ball and it hits the net and trickles over, players “apologize” with a hand wave.
New balls - When you are serving with brand new balls, you should raise the ball up before serving so that the opposing player knows that the balls are new.
Net smashes - If two players are at the net, and one of them has an easy slam, he or she is supposed to hit the ball around the vulnerable player. Hitting the other player is frowned upon.
Golf: Too many to list.
I can’t think of any for football and basketball.
But the best unwritten rule that I’ve heard is the following:
I was a watching an ESPN program a long time ago in which they showcased odd sports from around the world. There is a sport that is played between 2 opposing teams in Italy or Spain (I can’t remember which) only once a year. The object is to score a goal (like in soccer or hockey) into the opposing side’s goal. There are absolutely no rules. The opposing sides can kick, punch, bite, grab testicles, etc. No rules and no holds barred. It makes a violent hockey game seem like a friendly game of croquet. Players often have to be carried off of the field unconscious. The remaining players are all bruised and bloodied. Anyway, the one unwritten rule is:
You shouldn’t kick an opposing player in the head or in the testicles if he’s already unconscious.
Sounds like a good rule to me. Maybe they should write that one down.