The Rubber Stamp tool (symbolized by a stamp) allows you to select existing stamps from a dropdown list and apply them to the image. The usage is similar to the text boxes except that the stamp image will be applied instead of text. Stamps include Approved, Checked, Faxed, Processed, and Urgent, among others.
The Rubber Stamp tab in the Rubber Stamp properties dialog allows you to change the stamp to a different one after it has been applied to the image.