FreeCycle comes into my "junk" email account (the email account I've set up for that kind of thing) and I check it every day. I stop by GoodWill and Salvation Army on my way to do my weekly shopping at CostCo in my quest for Foley props. I've also found a great leather couch for my studio (only $50 at GoodWill!!!), and from FreeCycle several bookcases (which I put in the attic); I even got two boxes of honest-to-God 2" Auralex sound treatment - the color was hideous, but $600 of free sound proofing was the bomb!
Tag sales, garage sales and estate sales are also excellent resources.