The Video Game Critic

Perfect Dark N64
Battletanx Global Assault N64
Contra: Alien Wars SNES
Zelda: Four Swords Gamecube

That's all I can think of as of now. There are many more quality co-op games

Lego Star Wars (1 & 2) - Multi platform
Halo (1 & 2) - Xbox/Xbox 360
Earth Defense Force 2017 - Xbox 360

Plus, a lot of vertical scrolling shooters (Raiden III, Castle Shikigami 2, Ikaruga, etc.) offer 2 player co-op modes, though I'm not a huge fan of playing them this way - it often gets too confusing...

- Mark

If you have "golden age" classic systems, I deffinitely reccomend JOUST, MARIO BROS. and the 2600 version of SPACE INVADERS.

These are just my personal favorites:

River City Ransom for NES
Battle Toads Double Dragon for SNES
Any of the Metal Slug games
Some of the Seiken Densetsu/Secret of Mana games
Final Fantasy Crystal Chronicles for Gamecube

More as I think of them...

Gears of Wars on the 360
Capcom Compilation's pack 1 and 2 for some arcade beat'em up co-op goodness.
ditto the halo 1 and 2 recommendations as well
Midway Treasures volume 1 for Rampart pvp.  (i know it's not co-op but too good to pass up for battles between friends)

Here are my favorites

Gauntlet (Entire Series) - Whether its the NES version or the most recent PS2/Xbox game Gauntlet needs to be played with at least two people.  I personally love Legends (Dreamcast) and Seven Sorrows (Xbox/PS2). 

Contra (NES) - Almost any Contra game will do, but the orginal NES game is my preferred favorite for co-op.  For extra fun, don't use the 30 lives code and see who can last longer...

Metal Slug 3 - If you don't like Contra then be prepared to play Metal Slug.   I prefer the Xbox version, but you can play it on several systems.  Make sure you turn off the unlimited continues...it kills the fun factor.

Streets of Rage 2 - No co-op list would be complete without this classic genesis game.  Its all sorts of great and playing it with a friend only makes it better.

Zombies Ate My Neighbors - This is one of my favorite 16-bit titles.  Get it on the SNES or Genesis...it doesn't matter because both versions rock.  Excellent B-horror movie vibe with loads of humor!  I would kill for a decent sequel to this game (Ghoul Patrol sucks!)

Golden Axe - If you like hack n slash, then you really can't go wrong with Golden Axe.  Its a classic for a reason.

Guerrilla War - I could have gone with Jackal, Commando or even Ikari Warriors for the NES, but I really like this game.  You run...you gun...you have fun...nuff said.

Mortal Kombat: Shaolin Monks - Some folks hate this game, but my friends and I had a blast playing through this one.  I think its worth the $20 that it now costs at BestBuy.  Its like a 3D Final Fight, with loads of gore and Mortal Kombat lore...

Toe Jam and Earl - One of the few games on this list that is almost required to have two people to enjoy.  The single player is just, meh, but mulitplayer is a hoot!  Just a real fun game to explore and have fun in.

Zombie Revenge - This dreamcast game is one of my favorites co op games.  If House of the Dead was a 3rd person shooter it would be this game.  The dialouge is so bad its great and the gameplay is rock solid.

Thats all for now...

If you have a 4-player adapter for NES you HAVE to play RC Pro-Am II.

My favorite co-op games

Timesplitters 2/ Future Perferct
Basketbrawl (7800)
Centipede (7800)
Halo/ Halo 2

FUNK