My kids really liked the ones at Fort Wilderness, AKL, and Port Orleans Riverside, but I can't say they are any more special than most playgrounds you will find at community parks around the country.
That being said, just visiting the above resorts can be fun. I would not go for the playgrounds alone, but the other resort grounds and activities are really neat, and the playgrounds can be a fun play break for the kids.
Fort Wilderness has all kinds of fun, inexpensive activities, like viewing the horses, playing in the sand on the beach, taking a wagon ride, the Chip and Dale campfire, and watching fireworks on the beach.