I've only ever heard of oddment tables from one place - a Mr Ray Dillinger on the rec.games.roguelike.development group.. Here's the post in question.
Now I always thought this was pretty good idea but recently I decided it might be nice to implement it in a reusable fashion. First though let's quickly go-over what it is.
Basically it's a list and you ask for an item and recieve one at random. Each item has a different chance of being recieved, and this is called it's oddment. The item you pick remains in the list.
An easy way to think of this might be as a deck of cards. Shuffle the cards and pick the first card from the top. There are only two jokers in there. There are 56 non-joker cards and 2 joker cards. If you pick the top card you are unlikely to get the joker. A oddment table representing this situation would add each of the 56 non-joker cards with the oddment value of 56. Then you'd add the two jokers each with an oddment value of 2. It's that simple.
In a game, especially an RPG game, we might like to think of event cards or encounter cards. Let's say we had wilderness encounter deck, one might encounter rats quite commonly so we add rats with an oddment of 50 but a big box of treasure that's fallen off a cart is rather less likely, so we might put that in with an oddment of 1.