I subscribe to a magazine called "Orlando Attractions" and in the most recent issue, there was an article about discounted tickets to WDW, or other area theme parks. Like others in this thread have said, the bottom line was that any legit deal will only save you 5 to 10 percent or so. If you ever drive around the Orlando or Kissimmee area, for example, you may see those big balloons that say "discounted Disney tickets" or "we buy used Disney tickets" or something to that effect. According to the article I mentioned, what happens is that some of these people will buy your tickets from you, if you got an 8 day ticket and only used 6 days, for example, and they will then peddle these tickets to the public for a really cheap rate. People buy these tickets for a really discounted rate, but then are very disappointed when they get to the gate and the tickets don't work. One example given was that a father did this with his children, and then got to the gate only to have the tickets not work. He couldn't dissappoint the kids, so he was forced to buy real tickets at the gate. So now, he was out the money for the "discounted" tickets, and out the money for real, full priced tickets at the gate.
Three problems with these schemes: First, it is illegal to buy and sell used tickets in Florida. Second, there may or may not be any days left on the tickets. You may be buying a totally expired ticket, or there may be a few days left on it which will most likely expire soon, if the "no expiration" option wasn't purchased, but whether or not the tickets are legit is irrelevant because of the third point, which is that tickets (at least WDW tickets) are non-transferrable and are matched up with the fingerprint of the original user at the turnestyles. So even if it is a valid ticket with unexpired days, your fingerprint won't match at the turnstyles and you won't be let in.
So again, the bottom line of this article was that there are some ways of getting legitimate, discounted tickets, but most of the time, you'll only be saving 10 percent or less. Legitimate sources could include the Florida Welcome Centers, or certain travel companies which buy up bulk amounts of admission tickets and then re-sell at a discount in exchange for sitting through their time share spiel, or Triple A, as someone else mentioned. But I would avoid any balloon booths in the Orlando area, or internet sales, or anything else that doesn't look "official".