Double Dragon was originally released to the arcades and became an instant classic. It was the game that defined the sidescroller beat-em'up. The original characters names were Spike and Hammer, but this quickly changed to Billy Lee and Jimmy by the time it was released on other platforms. Fair Marion is outside of our heros' garage, when a bunch of freaky delinquents rush by and steal the steamy scream queen. The two poofy-haired brothers who are our heros run out of the building to see poor Marion being stormed away, and thus begins the game. Three buttons, punch, kick, and jump, and a joystick were your arsenal to rescue the fair damsel. Progressing through a series of screens kicking the crap out of everything in sight was the theme of this game. If someone came at you with a baseball bat, you could knock them senseless and then pick up the baseball bat to return the favor for a while. Same thing for whips, knives, boxes, oil drums, etc. Upon winning the game, if there were two players active, you'd get to duke it out man to man to see just WHO exactly was Marion's boyfriend.
After the original, there were four sequels made, as well as a Neo-Geo arcade box port. Double Dragon 3 was the death knell of the arcade life of the series, featuring abysmal gameplay and horrible digitized graphics a-la Mortal Kombat. The popularity of the game ensured its passage to the console world, where it appeared on the Atari 2600, Atari 7800, GameBoy, GameGear, Sega Genesis, Atari Jaguar, Atari Lynx, Sega Master System, Neo-Geo, NES, Super Nintendo, Turbo CD, and even the Playstation. The Double Dragon duo teamed up with the BattleToads for Battletoads & Double Dragon, which was released for Gameboy, Genesis, and NES.
I personally most remember the NES version, which had several additional modes which were what made the game my favorite. There was a Royal Rumble mode in which a huge group of baddies, a single baseball bat, and you were pitted in a free for all. There was also a two-player fight mode, where you and a buddy could duke it out one on one. The game had a hidden XP system, which was expanded upon greatly by the same development team later in the NES classic River City Ransom.
Double Dragon is one of the few games that had an enormous impact on the entire gaming industry for many years, easily witnessed in games such as Final Fight, Teenage Mutant Ninja Turtles, and even Street Fighter.