Trans Maro's story reminds me of a 15 degree night in the Manchester State Forest, three of us were sleeping in an enclosed trailer and had one of those radiant heaters that mounts to the top of a propane tank, it was the big double burner model.
About 3AM I woke up to a really strange noise, the heater was making this 'puttering' sound and was glowing orange. I pushed the side door open about 6" with my foot, it went 'whoosh' and started working normally. 
I turned the frickin' thing off.
When I bought my current house the AC system on the garage end of the house was "iffy" so I decided to do a split system and had a small unit added for the garage and den. It has worked great taking pressure off the big unit and the den/garage stay seasonal appropriate in temp unless the temps outside are extreme. Both the thermostat and cold air return are inside so the den is always comfortable. The garage is 73 degrees today when it is 90 and high humidity inside.
Insulation is also important. You can buy a rubber moulding that glues to the concrete floor for the door to fit into and it really makes a difference too.
The only time that I have ever had any complaints from my wife was when we did some fiberglass repair.
I have an infra red 30' long tube heater in my 20 x 40 shop. Keep the tempt set to just above freezing, and kick it up when I g oin. !5 minutes is about all I can stand under the thing until I go turn it back down. Cost to heat the shop last year was under $100.
These things heat the solids in the room; like the toolbox and car you are working on, rather than the air. Very handy for the weekend warrior where the shop sits cold all week.
http://www.schwankgroup.com/en/prod-tube-intro.asp
Cheap? Not really. Available used? Oh yeah! That is the way to find these guys and buy for $0.10 on the dollar. Just be sure that what you buy is correct for the fuel, either NG or Propane.
The other beauty is that you can paint in the shop without fear ob blowing up since it is a sealed unit that draws fresh air from outside and vets spent air to the outside.
TreoWayne wrote: I am in Omaha.
Another GRMer! I just moved here. PM me if you're interested in grabbing a beer and BSing over GRMey stuff.
I found that besides a space heater throwing a piece of old carpet on the floor helps keep the concrete from sucking all the heat out of you. I also manage to set myself on fire so often
Another Omaha GRMer here!
I use a propane heater and place the intake next to the big gap in my old garage doors in an effort to not kill myself. 
I put a CO2 detector on the wall, as well, just in case. It' heats it right up, but doesn't last long because of the lack of insulation. Usually it's long enough for short projects though.
I'm working on insulating and installing new doors for this winter.
I'd like to ditch the torpedo unit because the output still isn't really what a guy should be breathing. Thinking about a hanging unit and my gas line is on the inside wall so the plumbing shouldn't be too hard.
Don't forget, propane creates a lot of moisture as well.
Shawn
Well damn, they're coming out of the woodwork!
Wally wrote: I found that besides a space heater throwing a piece of old carpet on the floor helps keep the concrete from sucking all the heat out of you. I also manage to set myself on fire so often
But it keeps you warm.
I built a stacked can heater with a small computer fan and insulated tubing. It was free, and moved a decent amount of air warm air...
TreoWayne wrote: I have a cookie cutter house built in 2002 in the suburbs. It has a two car garage that thankfully has a single big garage door and does not have a pole in middle. I would say it measures 20x20' and the ceiling is maybe 8' tall. The garage door does not seem to be insulated. Two sides of the garage butt up against the interior of the house and one wall and the door face the cruelty of Mother Nature.
I GUARANTEE your garage door is not insulated. The cost difference is significant. Builders don't pay 5 times as much for a door for a cookie cutter house if they don't have too.
If you have living space over the garage, it is already insulated. If you do not, it is NOT insulated, and is your largest heat loss. It's easy to insulate.
The door is your second biggest loss. You can insulate (sort of) the existing one by cutting 1" styrofoam panels to fit into the door panels, and glue them in. You can also hang a big insulating blanket. Drapery material, heavy dropcloths, etc.
If you do these 2 things, you will conquer the majority of your heat loss. But you will still feel cold.
The concrete floor will make you feel cold. There is not that much heat loss there, but there is a lot of thermal mass. You need to separate yourself from the concrete. I use thick rubber floor mats to walk on and to lay on.
Once you have completed these 3 things, all you need is a little heat. I have used a torpedo style propane heater for years. I know they have safety issues. I run it for 10 or 15 minutes to get the chill out of the room. That's all I need.
Oh, and I also have an insulated shop style jump suit. Does the trick.
Here I am getting worried because I've been sweating so much lately that my sweat has no salt in it, and you guys are heating your garages. Bah!
Just a piece of cardboard makes a concrete floor feel comparatively warm. Lying on concrete in the winter just sucks the heat right out of you.
Radiant heaters that throw their heat right at you sure are nice and effective. Pretty cost efficient to boot. You're not really heating up the whole shop, just you and the area you are in. True, the rest of the shop will get heated, but most importantly, you get heated promptly.
when we were working out of my buddies detached garage, all we had was an old school hanging electric heater that was short cycling. Ran on regular 220 and kept the place warm. Of course having the place completely drywalled and insulated helped alot.
On those split systems, they are dirt cheap to buy. The issue comes later when something breaks. FWIW I am buying one to heat/cool my warehouse office. We'll see how it handles chicago weather 
Don't mess with your home HVAC system. It's sized for your house. Probably not enough excess capacity to heat your man cave.
Real men don't worry about fumes. Non-vented propane gas logs are in most new houses around here. I wouldn't have any problem using a propane heater in my garage.
I currently use a fan-forced kerosene torpedo heater, but my garage is VERY well ventilated, and I don't use it very often.
Ive been known to lite my little propane camping stove and let it sit on its lowest setting while Im doing garage work in the winter. Its costly and inefficient, but it gets the job done. A sheet of masonite goes a long way to keeping your butt off the cold concrete, and is slick - kinda like using a creeper too.
Any unvented combustion heater will have a problem that most people don't notice or recognize. MOISTURE. Combustion of anyting (gasoline, propane, or kerosene) produces moisture.
My wife decide we needed one of those fancy unvented prtopane fireplaces in our family room. We had one installed and it kept the family room (ex back porch) heated very nicely for the winter. But, by the end of the winter the drywall we had put on the ceiling (with 18" of insulation above) had all buckled and the windows were covered with a film that took her two days to get off.
Needless to say the fireplace was replaced with a vented propane stove for the next winter.
One of my BIL's has a 25X25 attached garage (2 walls and ceiling) that he heats with a large electric radiant heater mounted in the middle of the ceiling. It does an excellent job of keeping the garage warm.
In reply to Hal:
Moisture can be a benefit or a problem. During the heating season, our indoor air can get very dry. RH numbers in the 20% range are common. The moisture from the propane gas logs is a welcome by-product of the heat they provide. We don't use ours as a primary source of heat, though.
A couple of years ago I found a bunch of fiberglass celing tiles in a dumpster. A builder had finished up a subdivision and dismantled the job trailer. The tiles looked close to the right size so I grabbed them and put them in my garage door. It made a huge dfference in the winter and the summer. I was able to trim them just enough to fit inside the door panels, did ot need to glue them in. My FIL also finished the outside wall with OSB sheeting scraps from roofs, though I did buy the insulation we ut behind the OSB. The best part is it was mostly free.
Hey if you build a box 4 by 8 ish by 4 inch deep out of PT wood <aThen ;<< use a discarded single pane sliding glass door and you line it with black coroplast or garbage bags and pipe a solar fan at one end and have an opening at the other you have made a cheap ass solar heater . Face south ..
Two t-shirts, a sweater, and two pairs of track pants.
A 500W work light and get at it.
You guys are soft. I live a hell of a lot further north than most of you and I have no heat in the garage. (It is insulated though). 
I'd say dont mess with your house's heating system. You dont want fumes from your garage entering where your family sleeps! I'd suggest something like this: Propane Radiant Vent free Heater
We use a 30,000 btu to heat our 25'x25' two story cabin. Its fairly well insulated but for a small garage your could probably get away with 10k if your doors are insulated. Hang it on the wall, connect propane tank from gas station, plug fan cord in, and bask in the year 'round heat.
http://www.garagejournal.com/
You'll want to go there and check it out. There's lots of guys from canada, which is slightly cooler then your neck of the woods.
Most people on that board swear by the mini split systems. For heat and A/C, they are in a slim package and are available in single and multiple room applications.
http://www.ramsond.com/minizone.php http://www.minisplitsystems.com/
Only think about the heat or A/C after you insulate the garage though.
Odd...
Canoe without a destination??
This is the correct answer. Takes my shop from "I can see my breath to "t-shirt temps" in about 15 minutes.
https://www.schwankgroup.com/products/high-intensity-heaters/ecoschwank/
Runs on natural gas.
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