The Block Perimeter Applet is an excellent tool for building students knowledge of perimeters. Have them drag boxes from the left inside the simulation window to the green screen. After a number of boxes have been placed on the green screen count the edges of each box that are not touching another box. The number of free edges that you count is the perimeter. Now press the "Show Perimeter" button to check your answer with the computers.
NOTE: The boxes fit into the grid that contains the top, right hand corner of the box. To delete a box either drag it on top of another box or drag it off of the screen.