Many websites list "working" keys. These are usually keys stolen from volume licensing customers or educational institutions. Parallels actively monitors for these keys. When you try to activate the software, you will see:
Therefore, the "price" of a free key is often the sovereignty over one’s own data. parallels desktop free activation key
The only legitimate way to use Parallels Desktop for free is through their official 14-day trial. Many websites list "working" keys