Operation Flashpoint is a remarkable game. War simulator is probably more accurate a description than game. Levels come in the form of a handful of entire islands which you are free to stroll across which would probably take you a good hour to walk the length of each one. You can command groups of units, and are free to get in and out of any vehicle in the game including air-craft.
There are masses of mods and custom missions which give the game unlimited replay value. My personal favourite is co-op multiplayer. Spending a good hour sneaking through a forest with a silenced MP5 to find a platoon of tanks lurking on the other side.
At the time of writing the latest official version of OFP is 1.91. This requires the Resistance addon pack to be able to go above 1.46. The latest beta version is 1.96 (and doesn't require a CD). As I have made a fair-few missions and such, I thought I would make a small section on my site on mission editing for OFP.