Sandbox games have really advanced in the past few years, especially since the clones of GTA were released. Just Cause 2 is what I'd call the latest incarnation of the sandbox game. It is based on a fictional island nation in Southeast Asia, where you are looking for a rogue agent. Your task is to get in with the local revolutionaries and understand the big picture in Panau.
-Insanely large, multi-faceted open world
-Many different vehicles; planes, tanks, helicoptors, bikes, cars etc
-Graphics are generally good and very believable
-Pretty large weapons cache
-Grappling hook and parachute makes the game feel like a 007 movie
-Black market feature allows you to purchase vehicles and weapons anywhere on the island
-Game is difficult even on casual
-Ammo goes quickly, leaving you on a constant hunt for bullets (and health)
-Main storyline is really short, probably 7-8 hours...