Pandemic has released a free content update for Mercenaries 2 today, featuring four new gameplay modes - "Urban Rampage, Death Race, Arms Race and Mercs Blitz", according to an admin post on the official forum - and some "patriotic characters".
We're not really sure what any of that means. You can watch a trailer that might explain things, but it's mostly heavy metal music and things exploding. The content pack is called Blow It Up Again.
Or you could ask a PS3 owner - Sony's machine got the DLC update last Friday. There's no mention of it being made available to PC Mercs 2 players.
Note that the pack is "free throughout the rest of the year", meaning Pandemic could choose to charge for it or remove it soon, so grab it while you can.