The SEGA Genesis was a great system for co-op gamers and SEGA has done a great job in keeping the accessiblity to its titles available. Whether it's PC, XBLA, PSN or even iOS - there always seems to be a way to download a copy of a classic Genesis game.
Today Amazon is offering 48 Genesis games, bundled together for the PC, for just $7.49. The normal price of this pack is $70. You'll have access to the likes of Golden Axe, Toe Jam and Earl, and Streets of Rage along with many, many others. Here's the full list with the co-op titles linked.
The SEGA 48 Game Mega Arcade Pack contains:
Golden Axe Altered Beast Comix Zone Ecco the Dolphin Gain Ground Shinobi III: Return of the Ninja Master VectorMan Crack Down Shadow Dancer: The Secret of Shinobi Space Harrier II Alex Kidd in the Enchanted Castle Bonanza Bros. Columns Ecco Jr. Eternal Champions Fatal Labyrinth Galaxy Force II Kid Chameleon Ristar Super Thunder Blade Alien Storm Bio-Hazard Battle Columns III Sword of Vermilion Virtua Fighter 2 Ecco: The Tides of Time Decap Attack Flicky ESWAT: City Under Siege Golden Axe II. Alien Soldier Gunstar Heroes Landstalker: The Treasures of King Nole Light Crusader Shining Force Shining Force II Shining in the Darkness Streets of Rage Streets of Rage 2 Wonder Boy III: Monster Lair Sonic 3D Blast Dr. Robotnik's Mean Bean Machine Sonic Spinball Sonic the Hedgehog Sonic the Hedgehog 2 Sonic 3 & Knuckles ToeJam & Earl ToeJam & Earl in Panic on Funkotron