If you want premium Themes or Plugins for free then this is the site where you can download paid for themes and plugins that feature on sites like themeforest and codecanyon.
This is where some members on here are getting their giveaway downloads, now you can download the same for free yourself.

http://www.nulledthemes.net/

It goes without saying, do a virus scan. Files download as .rar, so convert to zip for free here

http://convertio.co/rar-zip/

Virus total scan them here http://virustotal.com

Further checks can be done with these below.

http://en-gb.wordpress.org/plugins/exploit-scanner/

http://wordpress.org/plugins/tac/

You should always test Themes and Plugins locally and not on live sites, so install localhost ( xampp ) on you machine. Tutorial below

http://premium.wpmudev.org/blog/setting-up-xampp/