Alien Breed is an excellent arcade-shooter released back in 2010. In fact, its original title was Alien Breed Evolution (only available on Xbox Live Arcade at the time) but after that it got its PSN and PC release.
You control a guy who tries to solve all kinds of mechanical issues on a ship while eliminating hordes of enemies.
Lots of shooting? Check.
Ugly, alien monsters appearing out of thin air? Check.
Few but very efficient weapons? Check.
Coop beside single player? Check.
Extra difficulty levels for die hard fans / crazy gamers? Check.
Great graphics? Check -> Hail to you, Unreal Engine 3, as always.
The only downside is the number of sections where you need to do some backtracking to operate a machine; this makes the available 5 levels in single player a bit longer without showing you new sections. Still, the environments are varied and interesting.
Cool little game: hours of arcade shooting fun guaranteed!
Worth the purchase? Check :)