I really liked the filament holders that used bearings for the spool itself to roll on but always thought they were inefficient and did not scale well since they always involved large prints and only held a single spool. I set out to fix this using minimal printing and cheap parts. This system uses a series of spacers and clips to hold multiple spools on 2x8mm smooth rods (the cheap $3 kind from the hardware store not the nice kind used for printer axis) Filament catches pass filament up to printer through PTFE tubing and hold filament ends for quickly switching as many spools as you want! (In my case 16) Spools are easily swapped without disturbing other spools.

UPDATE:
Video: http://youtu.be/BfAczR-4zKc
1.3 Added Filament catch system for easily switching between many filaments
1.2 Added cabinet rail holder and more photos.
1.1 Added smaller spacer to save print time when making lots of them.

Credit to LittleDangerous for the C clip

Sandwitch a standard 608 skate bearing between two spacers on one of the smooth rods and put a clip on each side, (alternatively, print spacers with 105% flow rate and the spacers themselves should be tight on the rod.) do this twice per spool per rod (a total of 4x per spool) I find that if I keep these assemblies slightly looser the spools spin better. Slide your bearing assemblies along the rail to account for different spool sizes. Insert small (or long) lengths of PTFE tubing through holes in filament catches and feed filament up through to extruder as shown in pictures. Use Dremel buffing bits to clean dust from filament.(Optional) Enjoy easy filament switching!