If you believe Amazon's ranking, I'd run around their site and check for their top selling products. I'd also look at their top rated, as I imagine there is a good overlap. They'll also have the product pre-categorized, which means half the work is done for you if you're trying to get industry-specific data. Given that Amazon is the top internet retailer, I would think they represent a valid sample of the internet as a whole.
As for trending data, I'm sure it exists, but not for free.