Mario Kart Double Dash - E It's Mario Kart, pretty self explaitory. It has Co-op mode just like all it's predicessors.
Timesplitters 2 - T Even though this game is rated T, it definately gets violent as you can kill zombies and parts of them can be blown off.
Mario Party 4-7 - E While this may not be what you are looking for these are way more fun with multiple people.
Pikmin 2 - E This game has gotten a little pricey but is worth it.
Star Wars Rogue Squadron 3: Rebel Strike - T this game is great becasue it has a ton of new levels and all the levels from Rogue squafron 2 are back and they are all Co-op!
007 Everything or Nothing - T This game has a fun Co-op and competitive mode. The only thing is there are some girls wearing very little or revealing clothing.
Super Smash Bros. Melee - T This is simple enough just play as a team.
Lego Star wars - E &
Lego Star Wars 2 -E10+ I have spend countless hours playing these games with family and friends. While some people think these games are for little kid, I be to disagree. These games have a great amount of humor in them and they a blast to play. (No pun intended)
Super Mario Strikers - E Even if you aren't a Soccer (Futbal) fan, this game is easy enough for anyone to pick up and play, and have an ejoyable time
I hope that is enough. All of these games except Pikmin 2 are $20 or under at Gamestop and you might find an even better deal online.