The 2nd DLC, Resurgence Pack, will be out on 3rd of June and will be an Xbox 360 exclusive for now, just like the Stimulus Package. It will be available on the PS3 and PC on 7th July, 2010.
The Resurgence Pack will cost Modern Warfare 2 fans 15 USD (same as its predecessor) and will include five maps, of which three are new ones and the rest Call of Duty 4 remakes.
Modern Warfare 3 (MW3) is expected to have at least three map packs, check out the Modern Warfare 3 multiplayer maps. If the DLC isn't your thing, prepare for Call of Duty 7: Black Ops which will be out on 9th of November, 2010.