There are commonly available drop-in LED upgrades for Maglite and Surefire P60-type lights
As Chaosmagnet notes, there are a number of LED drop in upgrades for common bulb types, like those used in a number of Maglites.
After many years of having various lights with fixed or proprietary bulb solutions that I realized simply became outmoded, I've made a concerted move to the P60 type lights. While the P60 format isn't quite a standard, there is a high degree of interchangeability and a fair number of vendors.
You can get lights that use the P60 format in AA configurations (although slightly awkward), 123A configurations and 18650 (rechargeable) configurations.
In addition, the P60 lamp assemblies can be had in a wide range of voltage options. You do need to pay attention because you aren't going to drive an P60 that wants 8v with a 2xAA configuration and you can blow a low voltage lamp that is intended for 2xAA with 2x123A.
My preference at this point is to get lamps that can handle both 18650 and 2x123A configurations. I primarily use them with 18650s, but have the ability to use primary lithium cells if needed.
FWIW, I really like the Solarforce L2P with the "forward" clicky tailcap, paired with an XPG P60 lamp assembly with the orange-peel reflector. It is an incredible value:http://www.solarforceflashlight-sales.com/product_detail.php?t=FB&s=7&id=112http://www.solarforceflashlight-sales.com/product_detail.php?t=RB&s=29&id=212
The XPG is a middle of the road LED right now. It's pretty bright, but has good runtime. the XML LEDs are brighter, but tend towards flood and shorter runtimes.
The lights from Solarforce are solid. The lamp assemblies are "OK".
Higher quality units are available by Nailbender on Candle Power Forums (CPF), or Malkoff or Torchlabs (the latter being available on overready.com).
I'd buy an XPG from Solarforce, but high end bulbs I'd probably go to Nailbender. I have one of the Torchlabs 3xXPG lamps and it is quite the lamp. Very bright, all flood, and very reliable.http://www.oveready.com/a-engines/cat_63.html