Kali Linux is Mostly used for penetration Testing.It is open source OS. It is absolutely free of cost. NO MONEY NEEDED. Other who use this will say that It will cost you money. BUT IT WON'T.

Every Linux is absolutely free. Some of them are not (depends upon the deveolpers that they want it to be free or not) UBUNTU is also linux and absolutely FREE. Download it from their site.

You can also download KALI from their site.

PLEASE SEED AND COMMENT IF ANY PROBLEM..!! :) :)