Outlaw (Atari 2600, 1978)

One of the first games designed by David Crane (of Activision fame), Outlaw is both a target shooting contest and a shootout game in one cartridge. When playing by yourself, you score points by shooting a moving target. When challenging a friend, the two of you (one gunslinger on the left and one on the right) shoot each other. 

Outlaw features 16 variations of play. In each of the games an object such as a cactus, a wall, or a stagecoach moves or stands between the gunslingers, or between the lone gunslinger and the target. In the two-player games, the first player to score 10 points wins. In the one-player games, you've got 99 seconds to score a maximum of 10 points.