A great, cheap, fast, and easy method of transportation for getting around the Mayan Riviera is via colectivo.
Colectivos are basically a cross between a taxi and a bus, and the cost is reflected in that. They go along specific routes between either Cancun-Playa del Carmen or Playa del Carmen-Tulum, and will stop pretty much anywhere along the highway to drop off and pick up passengers.
The collectivos are easy to spot - they're oversized white vans (that fit about 15 passengers) with either orange or blue writing across the hood and sides that say "Tulum - Playa" or "Cancun - Playa del Carmen" (or some variation of that) across them.
I heard somewhere that they operate until 10 pm, but I'm not sure if that means that the final one(s) departs each location at 10 pm or the final arrival is at 10 pm.
You can either take one directly from the pickup/dropoff point in Playa del Carmen (and specific pickup/dropoff points along the way) or flag one down on the road.
There are two main colectivo routes heading out of Playa del Carmen (North to Cancun or South to Tulum) and the originating pickup points are within about 100 meters of each other. They are both located on 2nd street ("Calle dos" - pronounced sort of like ki-yay dos), the Cancun dropoff/pickup point is at about 20th Avenue ("Avenida veinte" - pronounced sort of like vay-een-tay) and the Tulum dropoff/pickup point is about halfway between 20th and 15th Avenue. These are also the final stops in Playa del Carmen if you are coming from Tulum or Cancun. I do recommend taking a taxi to/from this point throughout Playa, especially if you are arriving at night.
The physical address (courtesy of Google maps) is: "Collectivo Pickup Stop", Calle 2 Norte LB, Centro, 77710 Playa del Carmen, Q.R., Mexico
If you are flagging one down on the street or the highway, simply stick your hand out as if waiving down a taxi, and if they have an empty seat (or a few) they will pull over and pick you up. They tend to flash their hazards on if they have room as they approach people on the side of the road. You may get one right away or you may have to wait a while before one comes along with room, we never had to wait more than 5 or 10 minutes even if the first one that came along didn't have room for us.
The cost for the colectivo from Tulum to Playa del Carmen is 45 Pesos each, and the price varies along the way depending on where you want to stop (for example, the price from Playa to Xpu-ha was 35 pesos each, and from either Playa or Tulum to Bahia Principe was 35-40 pesos each).
***You cannot take one colectivo from Cancun to Tulum or vice versa, you must switch in Playa del Carmen, however the point where you switch is within about 100 meters or so (30 yards or so) of each other.
Monkey photo taken at Coba