Whether it's an old door, a shelf you no longer need, or maybe even a section of floorboards, anything flat and broad can become a functional sturdy table with a set of these Tick table legs. They're basically furniture-making paperclips.
Made from a strategically bent piece of metal rod, the Tick leg clamps onto each side of a thick piece of wood without the need for nails, screws or glue. You just gently squeeze the legs together, causing the part that clamps onto to the table to expand -- which means it's ridiculously easy to install and remove without tools.
Depending on the size and finish you need, you can order a pair for around $US100 to $US120. So they're not as cheap as the stack of cinderblocks you stole from construction site down the street, but damned if they won't make your hacked together office just slightly safer.