Ben Chapman is a denizen of the Thingiverse, and his 3D-printed knife-sharpener is both inventive and ingenious. Obviously printed plastic won't yield a sharpening element, so Chapman designed the thing to fit onto the bottom of a standard coffee mug.
Why a coffee mug? Because as it turns out, the bottom of every mug has an unglazed ring of exposed ceramic, the part of the mug that's contacting the surface it's sitting on during the glazing process. Chapman's design allows a knife blade to contact that ceramic edge at the proper angle, honing the edge with a few quick swipes:
My only suggestion for a version 2.0 would be to create some sort of handle that keeps your non-sharpening hand above the blade, towards the dull side.
In any case, if you've got a 3D printer of your own, or access to a TechShop (which is where Chapman made his), the design can be downloaded here.