Within the parks: Walking is usually your best bet. MK has a train that has a few stops around the park, and in Epcot, you can take a friendship boat from one side of the World Showcase Lagoon to the other, but aside from that, there's not really any transportation in the parks that takes you to other parts of the park (with the exception of AK, where you can take a train to Rafiki's Planet Watch, but I'm not counting that, since the train is the ONLY way to get to Rafikis.)
From parks to other parks:
AK to all other parks: Bus
Epcot to AK: Bus
Epcot to MGM: Bus, Boat (from International Gateway Entrance), or Walk (International Gateway entrance)
Epcot to MK: Monorail to TTC and Monorail or Ferryboat to MK, Alternatively, boat or walk to Swan/Dolphin/Yacht Club/Beach Club/Boardwalk, and then bus to MK
MGM to AK: Bus
MGM to Epcot: Boat, Bus, or Walk
MGM to MK: Bus to TTC and Monorail or Ferryboat to MK. Alternatively, boat or walk to Swan/Dolphin/Yacht Club/Beach Club/Boardwalk, and then bus to MK
MK to AK: Monorail or Ferryboat to TTC and Bus to AK
MK to Epcot: Monorail or Ferryboat to TTC and Monorail to Epcot
MK to MGM: Monorail or Ferryboat to TTC and Bus to MGM
