Zero

Zero is hot because FREE is great. No one wants to pay for anything, and when there is a service or product you can get for free whether the service or product is a copy or similar in some way. People will pass on paying for something that has a few more details for a free service any day, but that goes off of preference of a product. For example, people will pay for Photoshop but free alternatives like Splashup, Picnik and Snipshop are just as good. They are not exactly the same as Photoshop with some of the features but serve the same exact purpose for free.