As above really, I have a spare wooden door. Nothing special, but quite nice all the same. Too good to throw away, but the house has a full compliment of doors already. I wondered about using it as a table top, but the recessions would make it a poor surface (not flat etc) so, any suggestions, creative or otherwise?
half doors used to make good jumps when i was a kid, just think you could have 2, all you need is a load of bricks! 
Find some spare wall space. Draw around the spare door. Paint the resulting rectangle black. Attach door to wall, so that it covers said black rectangle and is capable of opening. Invite your friends to see your new cellar. Hilarity ensues.
Use it in loft on rafters as a storage base or until you need it again
Just stick it on a wall somewhere to confuse people?
edit: camo16 beat me to it
I made a work bench out of an old door and some mdf it was alright for a cheap fix.
Open it to get into the spare room...
Put it up and leave it open, then it should disappear and turn into......
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IGMC
Buy a door sized piece of toughened glass, drilled at each corner and at mid-point down sides.
Get some narrow plastic pipe with a larger id than the holes in the glass. Cut the pipe to, say 100mm lengths (x6) and use as stand offs to mount the toughened glass off the door as a useable table surface. Id suggest some threaded bar, nuts and washers and rubber grommets to mount it all together, bolted through the door. The void between the glass table top and the door makes a good magazine shelf.
Get some stubby turned legs and fit to underside of door at coffee table height.
EBAY - shifted all my old doors this way. Not made a huge amount of money, but saved me running them to the dump.