As far as second-hand stores go, Savers is one of my favorites in general.
They tend to be cleaner, and priced more appropriately than Goodwill has been over the past few years.
I like I get my kids clothes here since they have this weird tendency to grow and wear out what they wear (go figure) so I try to take advantage of their daily Half-price special on a random tag color.
Every once in a while I'll get lucky and find a good board game (I'm a board game designer and collector, so I love finding oddities) or even a nice shirt or pair of pants that are (either practically or sometimes literally) brand new.
I think I even found a nice pair of hiking boots once when I was about to go camping and was in a pinch to just get something on the cheap.
Wash everything before you use it, of course, but I've had pretty good luck at Savers.