Salvation Army on S. Lamar has a good selection. There is a thrift store on S. Congress that has a good selection upstairs but I can't remember their name. Thrift stores
I bought my couch, coffee table and end table at Cort Furniture at Burnet/183. It's a furniture rental store but they also sell some of the used furniture for decent prices. There's also a really large selection there.
Cort Furniture is a great place - I got a really nice oak coffee table and an oak entertainment center for $70. Both are in great condition, are solid wood, and were cheap!