G-Force is a brilliant remake that plays very much like a flat 2D Tempest, in which enemies rain down from the top of the screen in an attempt to crash into your ship. Press the Z or Y key to shoot, and tap the X key to activate the special weapon.
Only two directional arrow keys are ever needed, and use of the superzapper is only allowed once in each level. Playing hints can be found in the same forum thread, though simple strategies can be formulated after a few plays. Highly recommended for a quick fix.