Reading the Ruler is a great resource for attempting to introduce the Standard Unit of Measurement, and the Metric system. It features the first two inches and also the first three centimeters. The student can slide his/her finger along the ruler to see what each mark equals. This will provide students with an interactive reference which would especially be helpful with those tricky fractional portions of the inch. Let me know how helpful this app is to your students.
(Click either picture to view in iTunes.)