A bearing based spool holder for 1 Lbs rolls. Works well for 1Kg rolls too!
get a pair of 10cm, m8 threaded rods, eight 8mm nuts and four 608zz bearings.

There are now two options for bearing guides, the second one is wider and will support other spools, particularly spools with thick flanges. Other than that, the length of your M8 threaded rods will determine spool compatibility.

Designed to clear an extruder like this http://www.thingiverse.com/thing:99274
and not cover up the cool "Prusa i3" logo thing.
Also because the roll is right over the extruder, if the filament tangles a little while unrolling, it won't pull the x gantry so much, as compared to a DIY spool holder made of things already in the room near your printer....

update:
after using this for months I just had a failed print on a 1 lbs. roll of bridge nylon. I believe this is due to me dropping the roll and having it unravel a bit before starting the print. seems the filament formed a knot as it twisted off the roll, causing the cold end to chew through the nylon. I'm going to experiment with a system to prevent the roll from unraveling due to loss of tension during retract moves.

update2: there's no easy/cheap way to prevent the roll from unraveling with this style of design. It would be much too complex. also it's not typically an issue: don't drop your rolls.