Steam's midweek madness sale this week is all about Dawn of War. All Dawn of War titles are 50% off - this includes the two player co-op Dawn of War 2 and the game's expansion - Dawn of War II: Chaos Rising. Another stand alone expansion pack is in the works as well titled Dawn of War II: Retribution, though this isn't offered at the 50% discount price. You can snag all three games, plus Dawn of War Gold, Dawn of War: Dark Crusade, and Dawn of War: Soulstorm all for $60.
If you do decide to pick up the orignal Dawn of War 2 or its expansion pack, you'll get access to the multiplayer beta of Dawn of War II: Retribution. While the beta isn't focused on the game's two player co-op campaigns it will give you an idea what its like to play as the Orks, Tyranaids, Chaos Marines and more.