I've been completely addicted to Counterspy on PS Vita lately. I can't put this game down. Perfect for the handheld and the most fun I've had playing games in a long time.
Kind of modern take on the side scrolling actioner, the game play is so much fun and the overall game design (by Mark Holmes) has Pixar written all over it.
Each level (and objective) is simple enough, and completing missions allows you to earn cash to buy new weapons and abilities.
And the weapons are awesome. The silenced pistol is a staple but I have to say I'm a huge fan of the Whisper (silenced) shotgun. Brutal but whisper quiet.