A pillow block for a bearing and 3/8" shaft assembly with pulley.
Edit 2013/09/30: I've rebuilt my motion control rig and the pillow blocks used to hold the bearings, pulleys and motor. The new pillow block design features an integrated, threaded bearing retainer. No more need to use 3/8" collars on your shaft to keep the bearings in place and your 3/8" shaft centered.
The new pillow block design also includes hex keyed through holes for mounting the block. This eliminates the need to use self tapping screws in the bottom of the block, which really don't hold up after repeated breakdown / rebuild cycles.
Lastly, the motor side pillow block includes an integrated motor mount for a NEMA 17 geared stepper. I got mine from phidgets.com but you can get yours anywhere. Hopefully the pictures explain how all these parts come together to form the basis of a belt driven motion control system.
I usually print my parts with MakerWare's standard settting, using 10 or 20% infill and default speeds. Nothing special, these parts print really easy.