I use two of these to mount my filament spool to the top of my OpenBeam Rostock. This makes the printer very solid and portable, I can just grab the whole thing and toss it in my car. It feeds nicely,with a curved filament path right into the extruder.

They would be generally useful in any place where you want to mount a rod at an angle to the beam.

As a filament spool holder:

1. For a triangular printer, the default 30% angle is appropriate.
2. Print a pair of these.
3. Mount on beams with short bolts, but leave them a bit loose so you can adjust position and get the rod in.
4. Print a pair of bearing inserts for your filament spool if you don't have them. Push a spool on bearing inserts. I used http://www.thingiverse.com/thing:7475, but there's tons of them now on thingiverse.
5. Put an 8mm or 5/16 threaded rod through the filament spool.
6. Place rod through angled pillow blocks.
7. Tighten pillow blocks.

The rod is friction fit, but it would be easy enough to drill a small hole for a grub screw to hold the rod.