Quantasm is an amazing ride that matches the name very well. I may have actually had a Quantasm while playing, as I believe I began to actually zone out because of the visuals.
You control a turret in the center of the screen that is being assaulted by all sorts of enemies. Some will rush straight at you, some will shoot you, some may circle you, and some may even become invisible when shot.
The real beauty of Quantasm lies in its drug-trip inspired visuals. The game looks to be in the vein of Geometry Wars, yet it has a lot more going on visually. Enemies let out huge shockwaves of particles when destroyed, huge multiplier names float upwards, and particles seem to fly randomly every-which way. The visuals are accentuated even more by the acid-trip music which feels like something out of a night club.
Quantasm is a quality freeware game that manages to be visually entertaining while still having good gameplay.