Easily earn free PMP PDU by being a volunteer

Under Category E for earning PDUs, you can earn free PMP PDUs , by just becoming a member of your local project management association.
You earn 1 PDU for each hour of service. The local PM chapters or association also conduct time to time sessions on Project Management related development and talks. You earn PDU for each such session that you attend.

Earn 30 free PDUs

Just found this and seems to be really good source for earning PDUs for free to manitain your PMP credentials -

Earn 30 free PDUs with The PM Podcast

All you have to do is just subscribe to their PM podcasts and listen through it for an hour.
Then simply claim 1 PDU for each hour of podcast that you have attended.

This can be claimed under category C of PMP PDUs.  The process for claiming the PMP PDUs have been explained at the above link.

Click here to see details about the categories.