Injen CAI. Completely and totally pointless, not to mention the aggrevating sound effects.
Autometer C2 gauges. I had the boost and narrowband air/fuel. The boost completely died on me after a few months, and the air/fuel was nothing but a waste of space.
Rockblocker tinted tail overlays. They were a total failure, despite the fact that I followed the installation instructions precisely AND had two people to help me. Explain to me how a flat overlay is supposed to go over a curved tail light without making massive wrinkles. Yeah. $50 that I'll never see again.
Innovate wideband gauge. I spent nine months -- yes, NINE months -- trying to get the goddamn thing to work. I was in contact with the company on a nearly daily basis, trying and trying to get it installed -- NOTHING ever got it operational. I finally lost my patience and told Innovate that if I didn't get my money back, BAD things would happen. To their credit, they refunded all my money immediately.
Mazdaspeed valve caps. Not a bad mod, but now I'm just bitter on the subject since they were stolen TWO HOURS after I put them on my wheels. ******* NJ punk-ass bitches.
And there's prolly more, but that's all I can think of off the top of my head.