Thanks to the two can crushing implosion instructables here, I've decided to post my own version. The best bottles to use are those with a thick or heavy wall. Two liter soft drink bottles are the worst kind, as are milk bottles. Gatorade or orange juice bottles work great.

A YouTube video (without sound) shows the process:

Step 1: Select a Bottle, Add Water

I prefer clear bottles, so you can see the water boiling, but it's not required.

Add a couple ounces of water to the bottle and place it in the microwave oven, without the cap! Put the cap nearby so you can get to it quickly.

Set the oven to full power, timer for three or four minutes.

Step 2: Grab It and Cap It

When the water in the bottle can be seen boiling, or when the glass fogs over from all the steam, open the door and put the top on quickly. Use gloves, as the steam will burn you badly. If you have a turntable, time the door opening so the bottle top is at the door.

Step 3: Air Pressure Does the Rest

The bottle will begin to collapse almost immediately. You can speed things up by running cool water over the bottle. Plunging the bottle in a big bowl of ice and water will really speed things up.