I like this one best -
Air intake "vortex" or "tornado:" It looks like a gerbil wheel and supposedly increases airflow (and thus engine power) by creating "vortexes" in the air intake system.
Some asian guy came up with that.
I was thinking of making one of my own out of a thin piece of sheet metal and some scissors. I figured it would save about $50. I never made or bought one.
That thing, at the store, costs 50 god damned dollars.
First time i saw a 30 minute infomercial for it, i called -
But if you try it, you need this gauge for your car -