Magnesium oxide is a popular drying agent. In powder form, it is hygroscopic in nature, meaning that it attracts water molecules to itself from its surrounding environment. In doing this, the magnesium oxide will pull those molecules away from other things, keeping them dry.
Libraries and other large book and paper storage facilities will often use powdered magnesium oxide to help preserve paper. Rock climbers often use magnesium oxide as a way to reduce moisture from perspiration on their hands and handheld equipment, though other mineral powders are also used for this purpose.