The humble hollow-core door is an unsung hero of frugality and ingenuity. Its basically a very plain, very cheap, wooden door that is available at any home improvement store for anywhere from $20 to $30 depending on its size. There are so many things you can do with these!

Work Table
Plywood warps, and MDF is heavy. But a hollow-core door is light-weight and is thick enough to withstand a lot of pressure before it warps. Better yet, they are typically made of unfinished wood, so they accept stain readily. Simply attach legs (you can find these at the home improvement store as well, or try IKEA's line of mix and match table legs) and you're ready to work! Make sure to sink screws or nails into the non-hollow part of the door or use anchors. Try adding a layer of protective covering like cork or a drafting board cover, and you can use your new table for all manner of crafty projects!

Headboard
Sometimes you have the money to spend on your mattresses, but not a lot left over for a headboard. And really, this is the way it should be! You won't be happy sleepingin a pretty bedroom on a terrible matress. Thankfully, hollow-core doors can provide a versatile and thrifty alternative to going bare. You can lean it up against the wall behind your bed, or even mount it on the wall if you prefer a taller look. You can stain it, paint it, or cover it with padding and fabric so that it looks upholstered!

Room Divider
Hollow-core doors come in a variety of sizes. In addition to the standard door widths (anywhere from 27" to 30" wide), you can also buy them for folding doors (widths of 12" to 15"). For a little more you can buy two or three of them pre-hinged, which makes them perfect to use as folding screens!

Bulletin Board
Every home office can use a large, organized bulletin board. Grab a hollow-core door from your home improvement store, lay a sheet of cork board over it, and staple a layer of fabric over it. Then hang the door over your desk or mail center! You'll love having all of that extra space to spread things out.

Magetic Board
Invest in a gallon of megnetic paint. Paint your door, then cover with fabric or another paint color! Stock up on small magnets.