They have also been made out of a wide variety of materials. It could be metal, enamel, tin, paper, or even wood. The first sign that comes to mind is this great advertising sign.
This sign is an advertising for the Kinney’s-Rome Company, and it's for their Deluxe Bedding that they made. You can see this terrific sign in my Etsy shop here. Another style of sign that is popular is an enamel sign, like this one.
As you can see, this sign is advertising Cyclone Fences, and this was actually attached to their fences that they put up for you. It dates to the 1930's and you can see this sign in my Etsy shop here. Signs can even be used in a store near a cash register to advertise a product, like this sign for Prince Albert Tobacco.
The great thing that this sign has going for it is that it isn't that big, so it won't take up that much room on the wall. You can see this Prince Albert sign in my Etsy shop here.
What kinds of signs have you run across?