Minecraft wiki entry for Placement, associated text: Pressure Plates
Signs
Torch (top, head, and back of piston only)
Cake
Lantern
Ladder (head and back of piston only)
Buttons (head and back of piston only)
Lever (head and back of piston only)
Vines (head and back of piston only)
Bell (head and back of piston only)
Sea Pickle
Banners
Tripwire Hook (head and back of piston only)