Medium Arkansas Pocket Sharpening Stone
This is the standard in sharpening stones, at least for the US. They are, as the name suggests, quarried in Arkansas, and work best when used with a but of light mineral oil or (slightly less messy) soapy water. Also as their name suggests, they are of medium grit--good for once you have gotten the nicks out and are actually re-establishing the edge. If money is tight, you could probably keep your knife "sharp enough" with only this stone and a couple of household items (think ceramic mugs, think car window edges)...as long as you didn't actually damage or nick the blade, anyway.
Current stock are smaller than pictured, approximately 1 1/4"x3"x1/4".