Q*Bert by Gottlieb, developed by Warren Davis and Jeff Lee. Arcade release in 1982.
I love development stories like this. To paraphrase.. "so, we decided to make some 3D-looking cubes on the screen. Then, someone suggested that a ball should be bouncing down the cubes. Neat! We decided to put eyes and feet on the ball.." Viola! And folks, that's how games get designed (seriously!)
And that was really the crux of Q*Bert. They had the genius idea of "jump on each cube to change it's color," clearing a board when then all of the cubes change. To take a cue from Pac-Man, two escape discs are provided which temporarily let you turn the tables on your enemies (time it right and they will chase you off a platform, plummeting to their death!)
Overall Q*Bert was just a very bizarre game. Its' concept was simple, but its' execution was very peculiar. For example, they messed with the fundamentals (the usual "up down left right" joystick was reoriented 45 degrees, and the entire game was played at an angle). Also, they played with the perspective a bit and some of your foes would travel along a different side of the cube (though your collision would still mean death).
Overall a very cool game, very early 1980s. For a time Q*Bert was right up there with Mario, Donkey Kong, and Pac-Man as one of the big recognizable characters in the videogame world - for some reason, he was never particularly exploited very much and while his brethren went on to big fame, he sort of stayed jumping on his weird cube pyramids. Maybe that's why we like him. Still fun to this day, and it gets quite challenging. If you can get past the odd control setup, you might find yourself getting quite sucked into this forgotten gem.