Super Smash Bros. Brawl, Mario Kart Double Dash, Mario Kart Wii, Sonic & All Stars Racing Transformed.

Not in your library, but TowerFall Ascension is an absolute must-have !

Games I'd like to play in couch Co-op are Legend of Dungeon and Hammerwatch, but you don't have either. Do you like Retro Gauntlet style games?

U have Dead Island and Magicka .

DBZ or Mario parties

From your library - Trials and Monaco both have local multiplayer I think.

Random PC suggestions:

• Party of Sin
• Mount Your Friends
• Broforce
• Sonic All Stars Racing Transformed

Jamestown

Speedrunners

Worms Armageddon!
And you'll get to create your teams, that's great fun.

From your Library:

• BattleBlock Theater
• Trine
• Worms Games

Not in your Library:

• Dungeon Defenders

In L4D2 it is possible to enable splitscreen.

You can also look here for some local coop games:
http://store.steampowered.com/search/?category2=24

Thanks for you suggestions so far, I'll look them up later tonight/tomorrow :)

Gauntlet, which just released on Steam and can be had at a mere $11.25 per copy by getting a four-pack on GMG with a 25% off voucher.

Risk of Rain

and mario party... if you hate your friends.

from your library Worms, it's a must play with friends

Mashed

Well worth a look, you may be able to get a copy for the xbox 360 (Original xbox game) very cheap second hand. 4 player on screen racing like Micro Machines.

Renegade Ops

1. Awesomenauts is my #1 for up to 6 people at once! I give it 9001/10 Stars. Small set-up time(Splitscreen,Controller Compatible/Highly Recommended,MOBA) Slight Learning curve

For Maximum Fun-filled memories you will need a few controllers and 2 decent(not a plain notebook with 2gb ram xD) computers & 2 copies of awesomenauts.
it was easy for me because I have the Microsoft wireless receiver and just used 3 of my xbox 360 controllers for one computer.
and for the second computer my friend had 2 wired xbox 360 controllers and someone just used mouse and keyboard.
just load up a private match of 3 on 3 and enjoy. if you have any questions add me on steam

2 Sonic and SEGA All-stars Racing Transformed 1-4 players (Splitscreen,Controller Compatible) Easy for pass and play

This game is a great racing game and can compared to the equivalent of Mario Kart for PC. Very easy for swap-in and swap-out play for multiple people.

3 on my list is Mortal Kombat Komplete Edition. 1-4 players (Controller Compatible) Easy for pass and play

2-on-2 tag-teaming with bloody graphics and crotch-kicking fun.
This is one that requires a decent computer to run to be able to have fun on.

4 Jamestown. 1-4 players (Controller Compatible)

It's a bullet hell game with the Jamestown history rundown all happening on Mars.
my first game on steam and it's one of the best couch co-op games out there for PC.

5 is Broforce 1-4 players (Controller compatible) Easy for pass and play

Great retro platforming with modernized action & you can enjoy all that while playing with your buds.

6 is SpeedRunners 1-4 players (Controller Compatible/Recommended)

great platforming race battling game, recommend xbox controllers for playing this.
give the demo a try! it's 4 players and this is a torrent published by the creators
http://thepiratebay.se/torrent/10616851/SpeedRunners_%5Bby_tinyBuild__amp__DoubleDutch%5D

7 is Castle Crashers 1-4 players (Controller Compatible)

Great RPG with lots to do.Not recommended for Pass-n-play but great local game.

8 Risk of Rain 1-4 players (Controller Compatible/Recommended)

very good Rogue/RPG game with slight learning curve, again not great for pass-n-play.

That's all the good ones I recommend. If you don't end up playing on a computer, go grab a Wii and load up some Brawl in tourney mode with items enabled :D

I really love battleblock theatre it is hilarious to play with ur friends

Garry's mod
Bro force

Gang Beast

Slam Bolt Scrappers. Its a sin that theres no online multiplayer ;_;

You should be able to get the Serious Sam games pretty cheaply.

Alien Swarm
Castle Crashers
Borderlands
Mortal Kombat Arcade Kollection

You should get the lego games
Playing Lego Marvel Super Heroes with my GF its awesome fun