This is one of those "hopefully this is allowed" posts... I've been selling on eBay since the late 90s, and I'm also a software developer. I wrote an application that I use to spot trends in specific categories on eBay, to know what to buy and flip for a quick profit. You can drill down into each category, and view the hottest items by # of bids, # of watchers, price, "popular items" (eBay's own mystery formula), plus see the popular search terms in the category.
The web site is totally non-commercial - there are no ads or affiliate links. I just figured I'd see if anyone else had a use for it... If you'd like to test drive it, let me know if anything seems unclear or awkward, or if there are some additional functions that would make it more useful.
The site is: http://www.listingtrends.com/