Magnets are a great way to provide information that you want people to keep handy. Use them to promote your company, your products, and your services. Because promotional Magnets provide helpful information, people are likely to stick them in convenient places like their fridge or memo board, so there's constant interaction with your message.
There's also a market for magnets with creative content, whether it be an inspirational message, pleasing graphic, or just fun.
Consider adding a QR code to your design to link directly to additional online information. This can be a great way to provide more detail than you would want to cram onto your Magnets. Since a QR code can link to an online message, you can change the online message without the need to reprint the QR code.