Scrabble - Whoever can make the longest word gets to go first. Also, we use friendly challenges... we figure we're playing for education so if you suspect someone's word don't be afraid to make them look it up. There's also Theme Scrabble where you get double word points for anything that goes along with the theme. In this case, theme-related proper nouns are allowed.
3-13 - If someone accidentally discards a wild card, you should let them know about it and give them the opportunity to pick it back up, once. If they do it again it's all yours. Also aces can be high or low but not both at the same time.
Train dominoes (aka, Mexican Train, but we just call it "train" because we're not aware of a non-mexican version). If someone leaves an open double and you rescue the table by closing it up, you get to take another turn. You don't have to take another turn but if you've got the tile go for it.
Also for all games, it's okay to leave notes in the scoresheet. If it was late at night, or someone was sick, or somebody cheated, it goes down. And we keep scores in specific notebooks so they live FOREVER.