Anyone who handles your luggage.
Example: If you use Magical Express and your carry-on luggage is too big to bring into the passenger compartment with you, or if you have strollers/car seats (the latter NOT being necessary or even usable if you are using only Disney transportation), the driver will stow those items under the bus. You would tip her/him when you retrieve your items at your resort. Customary is $1 - $2 per item, but not less than $2 total.
Example: You get to your resort and your room is not ready, so you leave your luggage with Bell Services. Diatribe alert! You tip the Bell Services person who brings your luggage to your room - not the one who removes it from your car, not the one with whom you first leave it. If, again, you've used Magical Express and let them handle your checked luggage, you will not need to be in the room to accept it; Disney takes care of tipping the Cast Members for that deliver; and if you have left any carry-on items with them and gone off to a park, those items will (well, should) be delivered to your room at the same time. If that's your scenario, you can tip when you hand over your luggage.
Example: SkyCaps. If you use Curbside Check-in at the airport, and even if your airline imposes a fee for that service, it is both customary and advisable to tip - minimum $1 per bag, not less than $2, there've been a couple of reports of especially good service when the tip has been $5 or $10 per bag. Conversely, you would NOT tip any agent inside the terminal who provides the same service.
Example: If you use Resort Airline Check-in for your return flight, the agents who check you in are the equivalent of the airport's curbside check-in SkyCaps, so you would tip accordingly.
--
You can OFFER a tip to anyone you feel has gone out of their way to provide outstanding service. Generally, they will refuse (although supposedly if you insist a certain number of times, they will accept). What you can do instead of tipping is to go to Guest Services at the appropriate park, or Management at the hotel/restaurant, and ask to fill out a compliment card. This goes into the Cast Member's file and is beneficial.