For those of you who have never heard of the site, it displays the most popular gear, gems, enchants, talents, glyphs, and now reforges, sortable by class and spec. It is designed to help players pick their talents, gear, chants, etc. by showing them what is popular for their class. However, us goblins can use it to help us figure out what's most profitable!
Back in LK, I would always check this site before I picked up a new gem cut. "Most popular" usually translates to "sells the most," and I wanted to be sure I wasn't buying a cut that looked good, but that no one wanted. And now that it has been updated with Cataclysm gems and enchants, you can be sure I'll be checking in constantly to help me figure out what cut to pick up next, or what enchant scrolls I should be making and posting.
Note, of course, that "sells the most" doesn't always equal "sells for the most," and this is another reason why I love WoW Popular. It's really tempting to run to the AH, scan the gems, and pick up the most expensive cut. However, that cut might be expensive simply because that's what it's listed for, not because that's what people are willing to buy it for. Low supply might raise prices, but if there's low demand, you might not see any profits. This goes back to what I was talking about a few days ago in the struggle between high profit at slow speed and low profit at high speed. WoW Popular can alert you to cuts you may have overlooked because of the low profit. Now that you know these cuts are popular and you can probably sell a lot of them, those low profit margins suddenly become more attractive! After all, selling ten at 20g profit each is preferable to selling one at 100g profit.
So go check out WoWPopular.com! Even if you're not a jewelcrafter or an enchanter, I'm sure there's something in there that will help you make gold! Find out what the most popular glyphs are, and sell them. Or find the most popular crafted PvP gear, and focus on selling those!