Hello,

Here's a rundown of the reprap 3D printer.  I apologize if a little of it is repetitive or too basic.  The full documentation is over at http://reprapdoc.voodoo.co.nz/ , and the project blog is at http://reprap.blogspot.com/

If you wanted to make a sphere using a hot glue gun, this is how you would do it:
You would bring the hot glue gun down to the table and draw a tiny small filled circle, raise the glue gun, draw a larger filled circle on the small one, and then draw a larger one again, repeating this process over and over until you had built up the lower hemisphere, at which point you would switch to smaller circles until you were done.

This process is called fused deposition modeling.
http://en.wikipedia.org/wiki/Fused_deposition_modeling

There are machines that do this, luckily, called rapid prototypers or 3D printers.  They're not cheap, perhaps CAD$30K, and the print cartridges aren't cheap either at CAD$6 for a cubic inch of ABS plastic.  One reason they're not cheap is that they're not common, and the other reason is that they're made out of metal by conventional machine tools.  Machine tools are expensive, machine time is expensive, and machinists are expensive.   This is a pity because 3D printers are really cool.

Adrian Bowyer's idea was to make a 3D printer out of plastic, called a reprap.  That way a hobbyist can take his or her nth generation reprap, print out a bunch of plastic parts, and hand them to a friend who assembles them into a new n+1th generation reprap.  This is much the way a potter makes a kiln from freshly fired bricks or a machinist makes a mill from freshly machined parts.  Or a computer program makes another computer program.  Luckily, plastic is fairly cheap.  (We're looking at a cost of ~$400 for a reprap, if we print the parts ourselves.)   We can get bulk plastic for $6/kg.  There's even instructions to make it from corn, and you can buy a ton of corn for $100.  (Disclosure: My wife is not hugely keen on my processing 1 ton of corn into thermoplastic in a basement workshop.  Even hypothetically.)
 
There's more on the plastics we're using here
http://reprapdoc.voodoo.co.nz/bin/view/Main/Polymorph
and here
http://reprapdoc.voodoo.co.nz/bin/view/Main/PLA

The heart of the reprap 3D printer is the plastic extrusion head.  It is basically a motorized hot glue gun.  Adrian Bowyer has a careful and detailed description of it here.
http://reprapdoc.voodoo.co.nz/bin/view/Main/Version2OfThePolymorphExtrusionHead
The plastic parts of the extrusion head are made from thermoplastic, printed by the previous generation extruder head.  They can also be machined using a milling machine, but that's not the point.  (By the way, the plans and everything else up on reprapdoc.voodoo.co.nz are GPL'd, like the Linux kernel or the rest of the GNU operating system.)

The motorized frame that moves the extrusion head is called a repstrap or reprap.  A repstrap is a reprap bootstrap, a carefully built motorized frame pieced together from whatever is handy in your workshop, spruce-wood, plastic cutting boards, or angle iron and threaded rod, perhaps even Lego or meccano.  A reprap is much the same thing, except most of the pieces are fabricated out of thermoplastic extruded by the reprap extrusion head.  

Perhaps the best option, certainly the easiest way to make a repstrap is to purchase or build a small milling machine, and convert it to use motors under computer control.  This is generally called a "CNC conversion".  I'm in the process of doing this with my Taig brand benchtop mill.  A CNC conversion involves scrounging or buying either stepper or servo motors, attaching them to the mill, and building or buying a controller circuit.  The controller circuit accepts commands from the computer, generally via the parallel port, and uses big fat transistors to order the motors about.  The reprap or repstrap behaves similarly, only instead of using a metal cutting head, it moves a plastic depositing head around, in effect doing addition rather than subtraction.  

What we would want to do in this case is remove the spindle from the mill and replace it with a reprap extruder head.

For control circuitry, we would use one of the reprap stepper motor controllers for each motor,
http://reprapdoc.voodoo.co.nz/bin/view/Main/StepperMotorController
along with the rest of the reprap circuitry.

We use the mill to move the extruder head around and print the plastic reprap parts.  Then we assemble them, slap the extruder head on it and some motors on it, take the circuitry off the mill, and we have a 0th generation reprap.   (And a mill, which is always useful.)

Regarding mills:  I've got a Taig CNC-ready mill (taigtools.com), which was ~USD$1200 without motors or circuitry, and overkill for this project perhaps .  But check out the Proxxon mini mill # MF70, which looks like it would make a great little repstrap.  US$ 275.82!  And it's so ... cute!  
For an entry-level homebuilt mill, look at the cutting board and dremel flex-shaft mill:
http://www.biobug.org/cnc/

Good resources for building or converting a CNC mill are the forum at
http://www.cnczone.com
and the CNC driver at
http://www.linuxcnc.org
The CNC driver takes a set of instructions, called G-Code, and tells the mill control circuitry, "OK, now move the cutting spindle down.  Now move the table left.  Now move it forward,  move it right,  move it back,  Now move the spindle up again!"  The G-Code is generated by the CAD/CAM program, where some human drew a rectangular prism on the screen.

In Ottawa, the best people to get in touch with are the Ottawa Robotics Enthusiasts, who host meetings the second Monday of each month over at Algonquin College.  They're currently more interested in building little autonomous sumo robots than cartesian fabrication robots, but they are very helpful folk who understand these matters inside and out.
http://www.ottawarobotics.org/
(Aside: Also check out Jin Sato's http://www.mi-ra-i.com/OPEN-JMM/ .  Sato is a former ORE member who built a beautiful benchtop mill, carefully documenting each step (in Japanese.))

There's also Rami Abielmona's excellent guide to Ottawa electronic component shops here: 
http://www.site.uottawa.ca/~rabielmo/electronicStores.html

In Toronto, the place to start is probably Active Surplus Electronics over on Queen street.
http://www.activesurplus.com/

Beyond that, I recommend checking out the Robot Builder's Sourcebook:
http://www.amazon.com/gp/product/0071406859/sr=8-5/qid=1147556899/ref=sr_1_5/002-9928436-5996057?%5Fencoding=UTF8

Also worth keeping an eye on is
http://www.dubsen.org/
which we may possibly use as a template for a reprap input file library.

-Sebastien Bailard
sbailard@dubsen.org