There are a few free shops in Berlin:
*Trial and Error Tauschladen- open Tuesdays and Thursdays 15.00-19.00 at Mareschstrasse 10 by S-bahn Sonnenallee. Mostly clothes (also for children). You can come and take what you want, bring what you want for free or for a small donation! It is a nice little community
*Leila- Fehrbelliner Straße 92: lots of clothes and lots of things you can borrow if you are a member for a cheap monthly price, i think a few euros (everything from bikes to tents to tools...)
*There was one right at the plaza on the east side of the Ostkreuz S bahn, I don't know if it is still there
There are a few more I am pretty sure, but I can't think of them at the moment.