Magnets are a great cheap way to decorate your fridge or metal filing cabinets in the office. I currently offer two kinds of magnets: sheet and neodymium.
Sheet magnets are generally going to be cheaper to make and can offer a greater variety of shapes that have more magnetic area available. However these kind are largely used for decorative magnets as they are not very powerful. They are unlikely to hold up anything thicker or heaver than a photo or a piece of paper.
The neodymium magnets I use can hold up to around 28 sheets of paper before gravity wins. If you want something strong or to be able to hold up a lot on a surface, this is it. These magnets are only ¾″ in diameter so larger designs will “float” off surfaces a bit and be unsupported. Designs for this sort of magnet should have at least a 1–1.5″ diameter area to hold the magnet.