FREE - In Google Play
FREE - in Win Phone Store
Here's a great app for practicing forks and skewers! helps a lot in improving your tactical vision by using Concentric Square Chess Vision Drill in Michael de la Maza's book Rapid Chess Improvement.
This program looks interesting
If you also want to learn more about forks, I recommend reading my blog post here:
You will get some free puzzles and understand how the fork works