Ninja Gaiden 3 Razor's Edge is coming in less than 2 months. It's only $40 and packs 4 playable characters and 100 online co-op missions in the Ninja Trials, and more. Also Dragon's Crown is coming with 6 playable characters, 4 players online co-op, and possibly the most jaw dropping, massive and replayable beat em up ever. http://www.youtube.com/watch?v=ym6VMkTV2-A http://youtu.be/V44wra_jQzE Not sure about Ramona Flowers vs The World online multiplayer DLC, that might come next month, and the main game is about $5-10 these days.
here's a video of Ninja Gaiden 3 Razor's Edge Online Co-op (Wii U version video, but PS3 version could play similiar. it also has a sweet practice zone while waiting for the mission to load). http://www.youtube.com/watch?v=bX1asgNeqGI
Resident Evil 5 and 6 are pretty fun in coop, Operation Raccoon City's coop is also pretty good, Dead Space 3's coop is really good, there's differences between the two characters you play as so you'll want to run through it twice, Call of Duty World at War's coop is ok, Dead Island can be really fun in coop, Aliens Colonial Marines is good in coop but the game is rather short.
I am a dedicated member of the "Walter Sullivan Is Bad-Ass" group!!! I am the true originator of the Cookie Demon theory on the SH2 and 3 boards.