I have to admit, I am a big fan of the "Rule of 2" we use for NIGHT SHIFT.
From the rules:
The rule of 2: this is my name for a sub-system in D&D that has never been precisely codified, but is buried deep in the bones of the game. Any time a situation needs to be adjudicated in D&D for which there is not another system, throw a die, and on a result of 1 or 2, it happens. Listening at a door (and not a thief)? You hear noise on a 1 or 2. Looking to notice a secret door (and not a dwarf or elf)? Roll a d6 and you find it on a 1 or 2. Surprise? 1 or 2. The only thing that changes, for the most part, is the type of die--rangers, for example, use a d8 surprise die--and some character types may adjust the probability (elves noticing a secret door without searching is a 1 on a d6).
A nice simple rule that covers a bunch of situations.