Mercenaries 2: World (Nearly) In Flames. It's exactly like the console versions of Mercenaries 2, except it's free! (And it's made in Flash. And it's two dimensional. But other than that? Exactly the same.)
OK, so maybe it's not as technically impressive, but we could think of many worse ways to kill some time until the real release than with this solid, old-school diversion. Enjoy.