Repurposed flashlights show up all over Trek. Let's try to catalog some of them here.
This is a wiki post. Feel free to edit it and then add a comment in the thread below saying what you added. Any and all help will be appreciated!
TNG Palm Beacon
"Identity Crisis" 09:33
Also used as an anti-addiction optical burst delivery device (roflcopter)
"The Game" 40:31
Auction listing for the TNG version: http://startrekpropcollector.com/trekauctions/item.pl?i=7712
And another made from a slightly different version of the Ready-Lite: http://startrekpropcollector.com/trekauctions/item.pl?i=10054
And a different one... is this the later one from DS9? http://www.melefx.com/mel/Gallery/Pages/Props.html#27
Source: First Alert "Ready-Lite"
There are two versions - one with a grid pattern on the lens bezel (pictured below) and one with a bar pattern. Both were used for beacons.
Star Trek: Generations palm beacons
Early TNG palm beacon
The first flashlights on TNG were introduced in Season 3. They were custom made from black acrylic with a lens made from machined acrylic rod. Inside were 3 Maglite bulbs, all powered by an external 9.6V rechargeable battery pack.
Source: Mike Moore http://www.therpf.com/showthread.php?t=70906&p=994110&viewfull=1#post994110
There were multiple versions, but one is this:
Source: According to Mike on the TPZ, the body was custom-built but the interior lights were Surefire flashlights.
Star Trek V Scotty flashlight
Source: a vintage flashlight made by the Garrity company in CT
@hms has a pic of a light that he calls a "VOY light tool" on his Flickr
Among other things, I believe this to be a likely source of the light diffuser used in the Troi hypnosis device:
"Lonely Among Us" 23:57
Anybody know where it was used in VOY?
Source: unknown. I had one as a kid, but no longer remember the brand. It had a small short cylindrical rubber button on the back for the power and the handle could be used to prop it up at an angle.
Source: Surefire M4
One lives in O'brien's tool kit but many crew members used one.
Source: Made from a Sunpak CZ-200A.
Mike Moore posted a photo of the add-on parts here: http://www.trekpropzone.com/forum/index.php?showtopic=6822
And a link to more photos is in this thread: http://www.therpf.com/showthread.php?t=70855