The PlayStation Plus subscription model has been a boon for co-op fans in 2012 with plenty of titles being made free as part of the package.  The latest is Anomaly Warzone Earth, a RTS/Tower Defence game that has an exclusive local 2 player co-op mode only available on Sony's console.

Anomaly: Warzone Earth has finally arrived on PSN, but why should we care?  Because it has two exclusive co-op campaigns that can only be found on the Sony behemoth.  The launch trailer is out and contains war, zones, but ironically very few anomalies.

Anomaly: Warzone Earth has been dubbed a "Tower Offense" game. Players form an assault squad and try to defeat alien defense towers. Simple enough. The squad is not controlled directly, rather a player chooses unit paths on the battlefield. In the PC, Mac, and XBLA versions, you could control a commander who would help advancing units by distributing power-ups. Alas, these were all single player affairs.