I constructed most of a 3D printer using the exceptional plans posted on Thingiverse based on the OpenBeam system. The link for the plans is here: http://www.thingiverse.com/thing:41967. The parts were printed on a Replicator 3D printer. Timing belts were ordered from beltingonline.com (T2.5 Standard Breco® Open Length Timing Belt, 6mm width, 2m length, Standard 590 White polyurethane, steel cables) as were timing gears (25 Tooth T2.5 Pulley, 5mm bore, 1off M3 Grubscrew Holes). I purchased linear shafts from McMaster (10mm, part number 6112K47 and 8mm, part number 6112K46). Couplers for the z-axis were purchased from ebay, while acme screws were purchased from TechPaladin. I installed limit switches on the front for the y-axis and on the right carriage for the x-axis.
Currently the "tool" is simply a sharpie pen.
The electronics wires were soldered onto an Arduino breakout board fitting over a Dueminalove. Power is provided by an old ATX power supply onto which I jury-rigged a power switch (ATX supplies are normally turned on/off by the motherboard of a computer).
I wrote a custom interface using the Processing language which allows me to move the pen between two points. If the pen is down, the software connects the two points with a line to mimic what should be happening on a real piece of paper. The video below shows the interface in action.
Link to Arduino Code
Link to Processing Code
My longer term goal is to mount a frosting extruder in place of the pen and use the Processing interface to allow custom decoration of cookies or something similar which could be placed on the platform.