One of my all-time favorite landlord-friendly products is this Picture Hanging Kit ($7.35), which you can find at your local hardware store. The museum-quality picture hangers are reusable and use pressure to hold artwork up, rather than actually nailing through your plaster or drywall. They can hold up to 100 lbs. of weight and only leave a pinhole. The kit itself can hang up to 17 works of art.