Update Oct 2017:

This Thing is here for archival reasons. In my own shop it has been completely supplanted by this system

https://www.thingiverse.com/thing:2362788

which I believe to be vastly superior.

Original:

Ever since my filament collection got past 3 or 4 spools I've been searching for the holy grail that would allow me to change filaments quickly without handling spools. It didn't take too long to figure out that if random access is a requirement, a separate holder for each spool is needed. That narrows the field pretty much to two approaches: holders on which a spool sits, bearing on its edges, and holders that involve an axle through a spool's hub. The former, in order to yield acceptably low friction, require ball or roller bearings. Since I wanted to accommodate 40+ spools, this approach was ruled out as too complex and time-consuming. I wanted something consisting of a few simple printed parts and a press-fit assembly process that would allow me to bang out 40+ holders in a few days. The TInkertoy-like result is presented here. Every single filament end is within arms' reach and I can change filaments without touching a spool. Only 7 different printed parts are necessary to cover most spool sizes on the market, at least the ones I use. These include 200x70mm (MakerBot, Prototype Supply), 160x90mm (Octave), 200x55mm (pp3dp, Bendlay), 220x55 (3dprintingsystems.com), Taulman, and others.

A really nice aspect of the system is that you can have multiple rows of spools, one behind the other, to maximize storage in a given shelf space. Simply feed the rear spools' filament under the front spool(s) and through its guide loop. One of the attached videos shows a rear spool in action. Use of rubber feet, especially on rear-row spools, is strongly recommended, to prevent them from creeping forward and touching spools in front.

Another great side benefit is that I've now removed the spool holders and spools previously attached to the printers, liberating at least a square foot of precious bench space.

The "funnel" attached to a gantry directly above the printer(s) is not strictly necessary, but it's convenient, keeps filament from dangling in front of my monitor, keeps all the filament ends in one place, and reduces the sideways force vector on the printer when its feeding spool is located some distance off to the side. The foam in the funnel cleans the filament, and more importantly, helps tame the unruly mass of protruding filament ends. Since it won't be easy to change the foam with 40 filaments going through it, you might want to have a separate, easily-changeable cleaner foam located at each printer.

Video of the FSS in action:

http://youtu.be/DE9xbywAdiQ

http://youtu.be/HNsTqzcEDbo

http://youtu.be/r5bAiAoVQeA

Full construction details are in the Instructions section. All printed parts are available on Tinkercad, search for "FSS".