I just collected some apps that has been gone free. If you want you also can check them on AppPicker.

–Flying Benjamins HDPrice $ 19.99 to free
– Quantum Galaxy HDPrice $ 19.99 to free
–T&L PolyglotPrice $ 1.99 to free
– Juicy Fruit PopperPrice $ 0.99 to free
–Blox 3D City CreatorPrice $ 2.99 to free

You can check all of the list here:​Top iPad games gone free for April 21st