I just got an older style Swedish military rucksack. The Newer ones look good too.
It's one of these:
http://theeverydaycyclist.wordpress.com/2011/02/06/swedish-army-rucksack/
I got it for under $20, the leather straps are a bit tough but nothing a little neatsfoot oil & elbow grease can't soften up.
I had it on my back with about 15 Lbs in it and it didn't feel bad at all. All the leather straps look like they'd kill your back,
but surprisingly they actually seem to do a good job of distributing the weight. I'm gonna make a few modifications
though. I'm looking forward to trying it out, I don't plan on packing more than 20 Lbs so it should be just fine.
I got mine from The Sportsman's Guide. You can also find Army surplus backpacks in Cheaper Than Dirt.
You gotta look towards the back of Cheaper Than Dirt, the front of the catalog is dominated by shooting
supplies, and The Sportsman's Guide has a lot of the surplus stuff online not in the catalog.