Love this idea! Take two IKEA Expedit bookcases, frame in MDF, paint, attach casters, and bolt some stained to the top. Designed by the amazing Justine Taylor. This would be good in craft room or an office and maybe a kitchen island.
Like shipping pallets, wooden cable spools are another item that is usually found for free on Craigslist. Since cable spools are limited by their shape there are fewer repurposing options, but they make a great coffee table bookcase.