What was the best automotive accessory or related infrastructure you ever acquired?.Mine was a freestanding carport.-Kevin
Air tools. 'Nuff said.
Factory Service Manuals
Ed B.
Good replies,take note sojourners-Kevin
My garage.
Good use of funds-Kevin
Voltage/ohm meter. Indispensable for working on today’s modern heavily electrical cars. Beyond that, I’m good with a Craftsman tool kit. Though I have to admit a garage/carport (or in my case, even just a driveway) would be nice.
Really, the internet is super useful too. Enables one to find hard-to-find parts cheap; look up service bulletins; second opinions through sites like this; and hook up with forums/support/tech info for rare vehicles. Its been invaluable to me.
Since I built up a large toolset over the years, I would say a GPS system to make driving in strange places easier.
Mid-rise automotive lift. http://www.bendpak.com/bendpak-products/specialty-lifts/MD-6XP.aspx
Tester
Garage heater in Minnesota.
Dollar for dollar, my torque wrench.
I bought it over 25 years ago, and because I always use it when mounting my tires, I have never had a problem with warped brake rotors.
Incidentally, the talk of service manuals brings to mind the Chrysler service manual for my '71 Charger. For several repair procedures, the first step listed was “Remove Engine”. However, nowhere in the manual did they list the recommended method for removing the engine!
The most useful hand tool I’ve ever owned, and I got my first one when I got out of avionics school in the Air Force in 1970, was and still is a speedhandle. I’ll never be without one.
Registering for CarTalk 
Electric 1/2" impact wrench, I have a 21 gallon compressor and this is faster and easier that using my air wrench , also I can run a cord out to my garden shed to take the blades or wheels off my riding mower.
remote car starter.
without having a garage, and getting lots of frost/ice/snow on the car, I find it helpful for getting the stuff off in the morning before I heard to work. It also helps on those humid, 90F(32C)+ days to cool the interior off a little bit when exiting work and walking to the car.
Just a trunk full of parts and new tires that came with a 72 Buick Centurion that I drove for three miles then stripped it and junked it. I sold the radiator and a disc rotor. I put the tires on my Cadillac, the stereo in my Lincoln, the speakers in my Impala, along with the new brake shoes. I eventually used the eight light sockets and the 19 bulbs. I found new battery cables, plug wires, four quarts of oil, two gallons of anti-freeze, two quarts of transmission fluid and a headlight in the trunk. It was the best car I ever bought.
Wow,thanks this is going places I never imagined and newcomers please peruse for ideas,these guys are plenty bright-Kevin