Just Cause 2 for £1.99 at the moment.
Cracking game, imagine GTA but instead of an upwardly-mobile crime-lord you're a terrorist (sorry, 'freedom fighter') on a South American island, one of the largest game maps ever.
The main 'thing' (all good games have a 'thing') that makes it stand out is that you have a grappling hook which you can fire at stuff and then either pull it towards yourself or reel yourself in. You can also use it to attach two objects together.
For example you can use your parachute (which respawns indefinitely) to parascend behind a car, or pull a bad-guy off a building, or attach the jeep you're being chased by to a truck travelling in the opposite direction, or jump out of a helicopter seconds before it explodes, fire the grapple at the other helicopter which just shot you down, reel yourself in, shoot the pilot, climb in and carry on your mission from in there.
I've been in a huge firefight, ran out of ammo, needed to take out a guy dug-in with a minigun and eventually got out of it by using the grapple to attach him to a passing aircraft. "ICANSEE myhousefromheeeereeee....."