The coda of my trip to Northampton was a visit to Webs.
From the outside and the first room inside it's fairly generic, though quite well-stocked. Then you get to the warehouse-like back room. OMG!!! It's like the Filene's Basement of fiber!!! We're talking Major Discounts here. Be warned: this is an extremely dangerous place for anyone with a serious yarn habit!
Aisles are organized by weights (bulky, worsted, sock, etc.) and skeins or cones.
Best to go with one or several projects in mind, so you can stay focused. There's an area up front, fyi, for non-knitters, with comfy armchairs and reading material, so they don't mind the wait while their knitting partners are in a Dionysian frenzy in the back room.