The generic "flashlight bulb" uses a PR4 mount. You can get drop in LED replacements at Home Depot or Frys Electronics.

I replaced the incandescent bulbs in our collection of old household 2D and 2AA lights with Dorcy 41-1643 (~1/2 watt, $7) LED bulbs, and dropped a 1 watt Nite Ize ($16)in my 2D maglite. Output was about 25-50 lumens and battery life was extended to 18 - 40 hours. Excellent around the house emergency and utility lights.

If you want more output, Terralux makes a couple that will also fit in the standard PR4 bulb mount. IMHO, they're too powerfull (140 lumens vs 30-50) for household lights. For reference, a 2AA minmag incandescent has about a 14 lumen output with fresh batteries.

The only thing to be aware of is the emitter in the LED bulb is not quite in the same place as the filament in the incandescent bulb and will change the focus of the light. Ours all went from spot to flood, and I couldn't get the maglite as tightly focused as before.