Information & Design's paper prototyping kit

ID's paper prototyping kit is a simple low-tech collection of user interface elements. The kit contains scaled GUI and Web screens, printed with a light grid. The screens can be printed on A3 or Ledger paper. The kit also contains labels of proportionally-sized controls such as fields and buttons. These have been designed for printing on Avery-size labels. You can choose between A4 and Letter sizes (the number of labels per page varies). Note that the source material is available in PDF format only.