you get 10€ for every 50€ you spend. works in germany only afaik.
it's explained here:
http://blog.de.playstation.com/2013/09/16/holt-euch-einen-10-e-gutschein-fur-den-playstation-store/
i loaded 150€ to my account (more is not possible), so i will get 30€ for free. this will get me some "free" indies for the ps4...
must-have-list for platforms i don't own yet:
WiiU: Donkey Kong
XBone: Dead Rising 3, Ryse