PDUs for PMI recertification? Do you know any good sites?

Hi,
I’ve posted a FAQ about getting PDUs (Professional Development Units) for PMP certification renewal. It’s here http://pmit.pl/en/project-management/how-earn-pdu-pmp-recertification-recertyfikacji/
I would like to add there good sites that give you opportunities to ‘earn’ PDUs for free or in a not expensive way. Do you know any good?
Best Regards,
Mateusz