In the hopes that this will help somebody else someday, here are the steps I had to go through to get this broken spark plug out. First, a quick recap: #2 plug was snapped off inside the cylinder head. Hex head, ceramic insulator, and the center electrode all came out with my plug socket, leaving just the threaded sleeve with the outer electrode still screwed in. I was using a regular 3/8" ratchet with a fairly long cheater bar on it when it snapped. This was on a car I had just purchased, and the plugs were only a couple years old, but the thing was frozen solid.

The crucial tool here is a size-5 screw extractor, known colloquially as an ez-out. It's basically a tapered, reverse-threaded drill bit. Stick it down inside the hole, and as you turn it counterclockwise, it screws itself in and bites into whatever you're trying to get out (they're meant for bolts with broken/rounded off heads that you then drill down through the middle of). I got mine at Ace hardware for about $4. The company that makes it is called Hanson, but I'm sure there are others. The end you attach your tools to is square, and it fits a 3/8" drive extention perfectly (though you have to put it on upside-down). Use a large crescent wrench to turn the assembly by clamping onto the male end of the extention. A 6" extention works well in this car. On a n/a car, you'll have plenty of room to work...turbos, good luck. I don't like the file/vise-grips idea that's mentioned elsewhere because I was afraid I'd break the file.

Mine was either corroded, cross-threaded, or over-tightened (haven't figured that part out yet) to the point that it wouldn't budge using only the above technique. I sprayed it with PBlaster 10 times over the course of three weeks and it still wouldn't move. Finally, I replaced the other three plugs, making sure to chase and clean the threads, which were a mess, then started the motor on three cylinders. NAPA sells a combination 14/18mm spark plug thread chaser for $7...you'll be amazed at how much crap comes out. Make sure you take your ez-out out of the dead cylinder before you start the car or it'll go flying. Also make sure your garage doors are open and your cigarettes are out because there are going to be gas fumes everywhere. It's also going to be really loud. Start the car and let it run for a few minutes. Turn it off and give it a shot of penetrating oil. At this point, I started using Deep Creep instead of PBlaster...who knows which one actually did the trick. Now spray the hole for several seconds with freezing spray, which you can get at Radio Shack for around $8. It's used for testing circuit boards. Temperature shock of the different metals (aluminum head/steel spark plug) to break the corrosive bond is the theory here. Give it another good, soaking shot of oil. Put the ez-out in, tap it gently a bunch of times with a hammer, take it out and spray the hole again with Deep Creep. Let it sit for as long as you can stand to wait, then put the ez-out back in with the extention and try to actually turn it out with the wrench.

I had to repeat this heating/freezing/tapping cycle three times before it finally gave. I waited about a half-hour or so between tries to let the different parts come back to equal temperatures. I feel pretty confident saying that eventually it WILL work if you're patient. Might have to give it an overnight to really let the oil soak in. I never had to use a cheater bar on the crescent wrench, and once it broke free, it was a real bear, but it got progressively easier to turn as it came out.

Hope this helps if somebody else gets stuck with the same situation.