Prague Thrift Store
Address: Šumavská 1050/29, Praha 2-Vinohrady 120 00
This place is in my neighborhood. Apparently it opened three weeks ago. They collect clothing/items from people and sell it/them. Part of their proceeds to goes to charity; the rest to their pockets. But hell, might as well let someone try to sell your free junk instead of just dumping it in a dirty cardboard box on a pissy street corner in Karlin. It's run by a friendly Hungarian couple who speak English well.
Re: Goods. There are two rooms. One contains old books/miscellaneous household items (irons, board games, an old skateboard) in addition to clothing/toys for children. The second main room is filled mainly with second-hand clothing. They have a decent selection of items for both men and women.
I wanted to buy a jacket but alas, they were all too bulky for me. I considered buying a second-hand iron (300 kr) and ironing board (500) crowns, but then I didn't really want to spend that much on something I don't enjoy. My girlfriend bought a hand-held hairdryer (200kr). She almost bought a copy of Le Petit Prince in Czech (100kr) but didn't (paperback = 70kr, hardcover =100 kr). Stuff isn't dirt cheap, but.. the stuff isn't complete dirt either. The lady said their policy is to sell stuff at a 50% discount off the original price. I guess whether that is cheap enough for you I guess depends on how badly you want/need the item.
If you join their Facebook group, you get 10% discount on your purchases.
There's an interview with the owner here, on expats.cz: http://www.expats.cz/prague/article/press-releases/prague-thrift-store/
Detailed details: http://www.expats.cz/prague/czech/thrift-stores/praguethriftstore-1/
Generally speaking, one of the better thrift stores I've seen here. And the people are friendly.
Second Hand Shop
Two weeks ago, I also visited an even larger second-hand store located on Blanicka street (near where it meets Francouzska). According to this googlemaps photo, the address is beside 768/4 Blanicka.
Although it looks like a small shop from the outside, it's surprisingly cavernous. 70% female clothing items, 30% mens. Check it out while you're in the neighborhood.