I had a pegboard in my garage with some unused space behind it. I wanted to make a cabinet for storing spray paint and for charging cordless drill batteries.
I cut out the pegboard in between two joists, gaining an extra 4" of room.
Added a 5/8" piece of old plywood for the back and side of the cabinet and sealed a small gap at the bottom with a piece of trim.
Built a fairly simple door out of the piece of pegboard I originally cut out, more 5/8" plywood and some scraps I had laying around.
Also lined the cabinet with some cut off carpet for insulation and snaked some power in there for a lightbulb.
The bulb will keep the cabinet significantly warmer during the winter to keep the batteries for my cordless tools warm and anything else I don't want to freeze.
That's it. Vote for me in the hidden cabinets contest if you like it.