This Alphabetical Order app is free today from ABCya. You can find out more information here https://itunes.apple.com/us/app/alphabetical-order/id548411302. It is only available on the iPad.
This app packs a lot of punch from an OT perspective. It encourages visual motor skills, letter identification and cause and effect. The app provides visual cues as well as auditory cues to match up letters or letter sounds.
You can play with the app several different ways. You can put the letters in ABC order, you can draw a line matching the letter case, the letter sound or the letter name. After you draw the line you touch the scissor to cut the chain and the letter slides down. It takes a few tries to get the hang of where to draw the line. It needs to be under the hanging letter so it will drop and slide to the correct answer.
You can also just play with the blocks stacking them in different ways. Draw lines and put the letters on the lines. There are many other open ended ways you can play with the letter blocks on the screen.