Not much else to say about this one, other than go download it now!
The demo here offers you a tutorial and a small-ish section of a scenario map named Towson, that I believe offers singleplayer and 2 player multiplayer. The demo clocks in at 265 megs, and from what I can tell from early reviews, you should download the demo at once, or I will be angry. You won"t like me when I"m angry.