PixelJunk SideScroller is a 1980's style arcade game made for the 26th-century. It's a side scrolling-shoot-em-up that exceeds in everything it attempts to pull off.
Between the style, the smooth controls and the soundtrack, PixelJunk SideScroller is everything I could have asked for in a downloadable game. Hell, it's everything I could ask for in a retail game.
You can pick up SideScroller on the PSN right now for $9.99, I would suggest you go do so.