Android: Per-app PIN locks are nothing new. However, they could be more secure. Dynamic Pin takes a page out of TimePIN's book and allows you to create dynamically changing lock codes for individual apps.
When setting up a PIN, you choose a particular type of maths problem to perform between two values in a randomly-generated matrix. You then use those values to make a four-digit PIN, which changes periodically. The actual four-digit number is never the same and you can't memorise it, nor is the particular algorithm you use to determine the PIN obvious.