Bohemia and 505 have popped a demo up for ArmA II on the game's official website.
In it, you'll be able to poke around the fictional post-Soviet state of Chernarus and the Chernarussian island of Utes. The tutorial boot camp missions are available, and the full single-player Trial by Fire level awaits once skills are up to scratch.
Tire of that, and the multiplayer map Mini Warfare is open to sample, with a choice of 20 vehicles, 25 weapons and various civilian and soldier models.
There's lots more in the full game, however, which we reviewed recently and thoroughly recommend.