Spare bedroom of my apartment. I have six bikes and need to pare down to four -- or make room for a different couple of bikes, dunno.
A friend who's a carpenter has a couple of bike rooms, converting a couple of those non-specific rooms in typical suburban homes: dining room, den/office/tiny spare bedroom. Racks for double decker storage. Pretty nifty. And he must have a lot more energy than I do. His bikes are always immaculate. I wash mine once or twice a year whether they need it or not.