Kali Linux 1.0 is a new and improved version of the famous BackTrack Linux.
BackTrack Linux is a very popular Linux distribution, that aims penetration and testing. So, after one year of silent development, the Offensive Security team has launched Kali 1.0.
Kali Linux is a Debian based distribution, using the Debian repositories.
To find more about Kali vs BackTrack, see this article, from the Kali official site.
To download Kali 1.0, go to this link and fill in the fields.