I've built a custom TV cabinet with storage for consoles, dvd player and dvd's/games etc, to fit in an alcove in the living room.
Large budgie cage 7 foot tall 5 foot wide 2 foot deep (waiting for budgie to die so I can have the room it's in as my arcade

).
An arcade machine out of an old bookcase and bedside table (in the early 90's wobbled like hell but like a dream to me) so I could rig my playstation and namco classics up with my twinsticks to play assault.
Multi Scart switchboxes.
Built a few wooden gates.
A small table out of 36 waterbottles and paper machee.
Currently building:
A supergun so I can play my board collection.
A cocktail cabinet (if I ever get time to finish it it will be a miracle).
A Darkroom for my girlfriends daughter in the corner of her bedroom.