I've been using MDF as the base for most of my cakes. I buy the large 4 x 8 sheet and cut them individually, the size I choose fits the cake. They're the perfect thickness to put fondant over and a 5/8" ribbon around the edge. This is very inexpensive and lasts a very long time. However, even a small cake on the board is quite heavy. What other choice do I have? Is there another type of board that I can put fondant on, is sturdy enough to hold the weight of a cake, thick enough to accept a pretty ribbon for the edge and lighter in weight?

I use cake drums.

I use foam core poster board. It is very light weight, grease proof and inexpensive (about a $2.50 for a 20" x 30" piece when on sale) You can buy 1/2" at Hobby Lobby or Michaels. However, it is very difficult to cut into a round, for an extra $1 HL will cut it for you into the correct size square.

I just stocked up on the foam core, like the PP said, if you catch them on sale you really can't beat it. I also use regular wood, cut usually in 12 inch squares and it never seems too heavy, I think the MDF is a little heavier than the pressed wood.