Free (as in beer) and always will be: Kali Linux, like BackTrack, is completely free of charge and always will be.
Details on what’s included are on the Kali Tools site.
More than 600 penetration testing tools included: After reviewing every tool included in BackTrack, we eliminated many tools that either did not work or duplicated other tools that provided the same or similar functionality.
Kali Linux is a Debian-based Linux distribution geared towards various information security tasks, such as Penetration Testing, Security Research, Computer Forensics, and Reverse Engineering.