What is the best material to use to line the bottom of a raised bed?

There are several options to use when lining the bottom of a raised bed. Everything from gravel to plastic. Options are discussed with pros and cons for each.

Related Videos:
Building Raised Bed
Starting a Square Foot Garden
Replacing a Wooden Raised Bed with Concrete Block
What soil should I use to fill my raised beds?

Related Resources:
Raised Beds
Raised-Bed Gardening