Most are available to everyone - to the GP (General Public). The only ones that aren't are Annual Passholder (AP) or there was a package for Disney VISA card holders only. I've seen Florida resident discounts in the past too, obviously you'd have to be a FL resident! Also some are
AAA rates, so you would have to be a AAA member to get that rate. The only way to find out is to read the boards and ask questions - or call Disney directly & they can tell you.
www.mousesavers.com also lists all current discounts & codes, and to whom they are available. Not all codes & promos are available at all resorts - you need to call Disney to check on availability for each specific promo. For example AAA might offer a reduced rate on every resort, but they only offer a certain amount of rooms at that reduced rate. Once they are reserved that promo is "sold out" at that resort, or even within that room type at that resort (pool view, standard, etc.)
My best advice is to call & call & call again! Even if the resort or room type you're looking for is unavailable, it may open up the next day when someone else cancels or changes a reservation on hold. This happens every single day, so it's not uncommon to call back & find what you want. People often have 4 or 5 reservations on hold while they firm up airfare & other details for their trip.