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 $100 to $120. 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.