These Knitting needles can have the following attributes customized:
type: straight, double pointed, circular
length
diameter (US,UK, JP gauge option coming soon)
number of needles
angle of the tip
sharpness of the tip

EDIT(19FEB2014): There is now an option for sliced/unsliced needles.

The needles have been cut in half to aid in printing on fused filiment fabricators (I will add the solid options soon). The idea is to print them, and then glue/fuse them together. Sanding or polishing will probably be required.

To make the circular needles, my idea was to insert a piece of printer filiment in the end and glue it.

I haven't tried this so, leave comments if you managed to pull any of this off.