They aren't really new, I think they first started coming out about a year and a half ago (early 2004).
I have the first one, Haunted House, which I liked a lot (there should be a review in the database here of it).
They're a combination of mini-games and adventures. That is, they are basically specialized variants of regular d20 meant for the included scenario - new classes and revised skill lists, relevent equipment, etc. For instance, the Haunted House one has 6 classes for psychics, and special psychic power rules. The one set in a school has 6 different classes for kids, I think.
I want to get the rest of the series, but they are sort of expensive for what you get (essentially a one shot) and my attempts to get review copies were unsuccessful. I plan on getting them eventually, but there's a lot of other stuff I want that would be more readily useful for me. They are really more suited to a pickup game or maybe a convention event.