New Zealands Op-shop directory is basically a website that lists all the op-shops in NZ, which is super useful if you're doing a roadie or just happen to be passing through somewhere. Also I managed to find some op-shops here in Wellington that I didn't even know existed. Plus it solves another issue I had, I didn't know when the stores open and shut. A lot are closed on Sundays, and there is nothing worse than taking a bus trip all the way to the other side of town to realize the shop you were going to is closed.
I'm not too sure if other countries have the same sort of thing, but if little old New Zealand at the bottom of the world does, surely other places would? And it's definitely worth a look because the more shops you know of the better your chances are of finding some great stuff.