Step 1: Get a Rock. It should be vaguely flat sandstone.
Step 2: If the rock isn't extremely flat, find another rock and rub the two together until you have a flat surface.
Step 3: Rub your knife's edge against the flat part of your rock.
Step 4: Repeat as necessary. :thumbup:
This will get your knife 'decently sharp', generally speaking. I'd say this system will give you the best sharpness-to-cost ratio out there. Cost: none, Sharpness: Decent.
Recurves will be a problem, though. For that, you'll want to look for a rock with a curved edge...
In all seriousness, though, sharpening free-hand is a pretty inexpensive, simple way to sharpen most knives, and stones come as fine as you like.