Valiant Comics (or Valiant Entertainment as they're known these days) were a big thing in the '90s and came up with many slick gimmick covers, including chromium covers. They went bankrupt for a bit after being bought out by video game company Acclaim, but have been back for almost 10 years now and have introduced several new types of gimmicks.
One of the coolest is a cover made out of carbon fiber:
I had won one recently by auction but the seller cancelled the order, probably because the price I won it at was so low compared to market value. Oh well, I'll get one eventually.