THANK YOU VERY MUCH, Mickeep! I had no idea there was such a bus. This will make our hopping so much easier!
There is direct transportation from each park to all of the other parks (although park buses at MK are actually at the TTC, not at the park gates themselves).
MK-Epcot: Monorail (transfer at TTC)
MK-DHS: Bus from TTC
MK-AK: Bus from TTC
Epcot-MK: Monorail (transfer at TTC)
Epcot-DHS: Bus from front of park, boat or walking path from International Gateway entrance
Epcot-AK: Bus from front of park
DHS-MK: Bus (stops at TTC, not gates)
DHS-Epcot: Bus to front of Epcot, boat or walking path to International Gateway entrance
DHS-AK: Bus
AK-MK: Bus (stops at TTC, not gates)
AK-Epcot: Bus to front of park (no direct transp to International Gateway, but you can take a resort bus and walk to IG from an Epcot resort)
AK-DHS: Bus