Anything and everything. Some of my top level download folders:

Aircraft and Spacecraft, Buildings, Camping and Fishing
Christmas and Winter, Diving-Scuba, Equestrian, Food, Furniture
Guns, Gym & Gymnastics, Halloween, Interiors, Industrial
Japanese, Medical, Medieval, Military, Old Time, Plants, Pool-Swim-Beach
SciFi, Sea-Underwater, Ships and Boats, Sports, Toys and Toon, Western, Vehicles

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken