Boomerangs is a fantastic charity shop located in the center of hipness in Jamaica Plain. Their sales benefit the AIDS Action committee and they have a fantastic selection of clothing and other things to choose from.
I go mainly for the clothing, but I have friends who shop there for furniture almost exclusively. They get fantastic stuff in! Recently, before a wedding, I managed to score an Ann Taylor dress, with the original tags still on it. It was originally $79 dollars....at Boomerangs it was $8!
The staff is very friendly, the clothing is well stocked, well marked and sorted and wonderfully priced. The stock changes quickly, so it's best to visit often.
There is another location of Boomerangs in Brighton which has many toys and books for children. Boomerangs also has an auction site occasionally, where you can bid on great items. All in all this is one of my favorite stores on Centre Street, and I always walk out with something great.