Steam is having a weekend sale on Dawn of War 2 and Dawn of War 2: Chaos Rising. The two player co-op RTS from Relic was one we certainly loved and the expansion pack was equally as entertaining. If you haven't picked up either yet - you can snag both in a Gold edition for only $20, or you can snag the expansion pack for $10.
Seriously - if you haven't picked this up yet and you love yourself a good RTS/RPG hybrid - buy it now. There's a new expansion pack for the game hitting this spring, but as of now, co-op is unconfirmed for it.
Source: Store.steampowered.com