Replace the bulb in a 2xAA mag-lite with a bright white led. A cool burning led will give you longer battery life and won't burn your face off.
Step 1: Collect the parts
You'll need a 3.2 volt LED -- I think most bright white LEDs qualify. A ruler or caliper. An old, beat-up mag-lite, the kind that takes 2 AA batteries. Some wire cutters. A low speed or variable speed drill and 1/4" bit (not pictured). A small vice or pair of adjustable, locking (vice-grip style) pliers will come in handy (not pictured).
Step 2: Disassemble the head of the flashlight
My old mag-lite was already missing the bulb. Unscrew the front part of the head to gain access to the parabolic mirror.
Step 3: Drill out the hole in the plastic mirror
Place the plastic mirror carefully in a vice, hold it with a pair of slip-joint or vice-grip pliers... or if you have the hands of an oil-rig worker, with your fingers. Slowly drill out the center hole of the mirror with a 1/4" drill bit. This will allow the mirror to fit over the LED.
Step 4: Trim the LED leads
Trim the LED leads to 1cm, .4 inches, or 3/8". You could probably go a little shorter to put the LED closer to the focus of the mirror, but this spreads out the beam the way I wanted it.
Step 5: Discard mag-lite bulb and install LED
Wiggle the LED around until you can insert the leads into the two small holes that used to hold the mag-lite bulb leads. The LED should light up if the batteries are installed. If not, flip the LED around. Remember, this model of mag-lite is turned on by unscrewing the head of the flashlight.
Step 6: Screw the head back on the flashlight