Shape Surveyor
This interactive site will give you some practice calculating perimeters. It is a bit elementary, but if you choose the "super brain" category you can practice finding perimeters.
http://www.funbrain.com/poly

Constant Perimeter and Area
This interesting site examines the relationship between area and perimeter.
http://www.edc.org/MLT/ConnGeo/CP.html

Perimeters and Area
These sites give some practice calculating perimeters and areas.
http://www.onlymath.com/perimeterRectangle.html