I just picked one of these up last week and will be using it to haul home a 10' sheet metal brake from a local rental place this weekend (and for many other things I'm sure):
~$30 from Harbor Freight.
Truck Bed Extender
I haven't used it for 8' plywood yet, but I have an idea as to how to make it work great for that; out of the box it's only really good for stuff that's 10'+. I'll be building some cornhole boards in a few weeks with some friends of mine, so I'll post pics when we go to pick up some plywood then.
Just an FYI, the one that HF actually had in the store was not identical to the one on the website (they source from all sorts of asian manufacturers)...mine only has about 7 inches of adjustment. On the lowest setting it's perfectly flush with the floor of the bed. on the highest setting it lines up pretty well with the top of the wheel wells. Yes sometimes HF stuff blows, but this looks like it'll do enough for what I need and without seeing other manufacturers stuff in person to see how thick the metal is, the design is, for all intents and purposes, identical.
Also, HF doesn't advertise it this way, but you can install the short end of the large L-shaped piece in the receiver and use it as a ladder rack in conjunction with your roof rack if you have one.