TMA's trolley service that, starting Dec 15th, operates free 7pm to 12:30am. You can find more info here:
or visit the TMA's website here:
Ski shuttles are always available in ski towns, but the catch is knowing their schedules. My family rented a car for the week so that we could visit different area ski resorts. One reason for this was because we were staying in a house, not a hotel. If you will be staying in a hotel you should have no problem with catching a shuttle to the resort of your choice. I recommend Squaw Valley!