According to the Disney website:
Complimentary transportation throughout Walt Disney World Resortincluding transportation to and from Disney Theme Parks and Disney Water Parksis available to Guests of select Walt Disney World Resort hotels.
But then the FAQ's say this:
Q. Im not staying at a Disney Resort hotel. Can I use the monorails, buses and boats to get around Walt Disney World Resort?
A. Yes. All Guests have complimentary access to our network of monorails, buses and boats. If you would like to visit multiple parks, enjoy a nice dinner at one of the Disney Resort hotels or shop at Downtown Disney Area, you can leave your car parked in the theme park lot and use our transportation network.
While there are many ways to get around Walt Disney World Resort, here are several recommended routes that may be convenient for Guests who are not staying at a Disney Resort hotel, but wish to visit multiple parks:
Magic Kingdom park and Epcot are connected via monorail
Epcot and Disneys Hollywood Studios are connected via boat
Magic Kingdom park and Disneys Hollywood Studios are connected via bus
Disneys Animal Kingdom theme park is connected to the 3 other theme parks via bus
Learn more about Walt Disney World Resort transportation.
So it sounds like you can go from park to park on the buses and to the resorts and back to the parks. But parking at a resort and using the bus system to avoid paying for parking is not intended.