Okay, more rules:
A highly amusing and moderately painful solution to the slapping problem (e.g. if you know that your next card is going to be slappable, you can immediately slap it as soon as you put it down) is to make it so that a player has to slap his head before he can slap the deck. This adds a delay factor, and, as an extra bonus, you get to see a table full of people hitting themselves in the head. Fun for the whole family.
If three sixes are played in a row, on top of each other, play stops immediately and the deck must be burned before midnight, local time. This will rarely happen with standard rules, unless nobody notices the first pair.
Any sequence of two cards that adds up to 10 may be slapped. Tens may also be slapped. Face cards break the sequence (i.e. 4-J-6 cannot be slapped), but aces count as one (A-9, or 9-A, can be slapped).
A variant I invented and, for unknown reasons, titled Gecko goes like this:
Whenever somebody wins a game, he may either (a) create a new, fair rule, or (b) remove any rule. It helps if you keep track of the rules on a sheet of paper. I usually start from very basic rules when playing Gecko, such as only slapping on doubles. It gets complicated soon enough.