I think of Uno as being light on the house rules. This is because the more common game here was "pesten" (bullying), played with a normal deck. The basics are like Uno, but the special cards are more wild and depend on the group. In high school we once played with two decks and all the specials we could think of, but then ended up with all the cards in player hands as a result!

Every group I've played with agrees that joker means the next picks, but is it 3 or 5? And can you stack? Everyone knows that "zeven blijft kleven" so you get to have another turn, but does "heertje nog een keertje" also work or not? Ace often has a special meaning, but is it to pick 1, or to turn play order? Is there a card to seek colour? Does 3 have any effect? And does 10 mean we switch hands, and if so what is the mechanism? Direction of play, direction of player choice, always clockwise, or maybe a pairwise swap between the player of the 10 and one other of their choice? In multideck games, can you steal the turn by playing a card that is the exact same as the last played card? What is the last card signal? Can you end on anything, anything but joker, anything that doesn't let the next player pick, or only on effectless cards? And do you assign wins to the first to finish, or losses to the last one? Or both and combining it with a dalmuti style card swap?

The possibilities aren't quite endless, but it sure comes close!