Mercenaries is a 2005 action game by Lucasarts and Pandemic Studios. It is available on PS2 and Xbox, although this review will specifically refer to the PS2 version.
Mercenaries puts you into the shoes of one of three mercenaries dropped into the middle of a fictional near-future war between the Koreas. You then run missions for the various factions, gaining intel to capture the "Deck of 52." Although you are given three players to choose from, the differences are almost unnoticeable, so your choice is mostly cosmetic.
The quickest way to describe Mercenaries would be "Grand Theft Auto North Korea." The game uses a similar free-roaming system, and like in GTA, you can simply ignore the missions and raise hell. This is extremely satisfying, as everything blows up when you hit it with enough ordnance. Overall, Mercenaries is an excellent game, especially for anyone looking for a more pure-action version of GTA.