Paint FX by Sprite Labs
Masking has become a common feature with photo editing apps. The success of this tool largely depends on the user interface and how well it's integrated into the overall experience. Paint FX makes masking appear seamless and effortless. Simply apply a mask, select a filter and its immediately applied to the masked area. To add more effects, it's best to add a new layer, which will subsequently merge the last two. Paint FX is $1.99 in the App Store.