Save the date cards can take the form of any message medium, and often do. They sometimes correspond in style to the wedding invitations, or they take the form of refrigerator magnets, brochures, postcards, booklets, or tri-fold letters. The medium is up to you, but the shortness of the message usually lends itself well to magnets or small cards.
There are many types of save-the-date. Although a save-the-date is traditionally a card, it can also take the form of magnets, bookmarks, or wedding stationery. Any type of save-the-date can be a meaningful keepsake of your wedding day. They also serve the purpose of giving your guests the first indication of what your wedding will be like and what to expect at the wedding reception.
Some companies offer digital save the date cards as well. What this means is you have the card to use on your computer and you could send them out to any email address that you have. Since almost every household has a computer and an email address, this would be a good way to save money on postage stamps.
You may also consider doing customized printing on objects that will grab attention. For example, a pinwheel or a fan.
Another well received way is to print a mini calendar page with a 3-hole punch so your friends can attach it to their own calendars. Of course, you would have to circle or decorate the day that belongs to you.