It is called Explania.com and it has a growing range of excellent animated explanations, interactive tutorials and instructional videos.
These animations are not only well made but are made free for anyone to use via a creative commons licence. Teachers can use them in class or even embed them within their websites, blogs or wikis.
I really hope they get a lot of support and their selection of videos continue to grow.
I have embeded two videos from their website below.