Light. It's one of the most important "resources" - lack of light won't necessarily kill you, but it sure is limiting. I consider flashlights to be one of the most valuable tools in a disaster - they enable tasks to be performed in the dark and are powerful security tools. Below are some of the lights that I've selected over the years.
As a general rule, in selecting a light, opt for quality, battery life, and reliability over high lumen counts and large numbers of modes. In fact, I've basically stopped using all lights with more than about three modes (High, Medium, Low). Further, most flashlights will have a radically inaccurate lumen rating. Most Chinese flashlights that claim a specific brightness are off by a factor of 5 to 10.
Now, if you want a security or defense light, you may want to get a light with a strobe mode (but not mixed in with four other modes - it won't do you any good if you can't find it when you need it) and really high brightness.
Amazon sells a cheap, $4 flashlight that uses a single AA battery. It uses an LED bulb and is relatively bright. I almost wish it was less bright so it would have longer run time. These have proven pretty durable and long lasting and are well worth the price and beat out lights quite a bit more expensive.
A more expensive LED light ($17) that has worked great for us is the Gerber Ininity. That light last forever on a single AA, which makes it a great basic task light.
How to build an awesome flashlight for about $25
I've used and carried a lot of lights over the years, but right now my EDC light is one I build myself. Well, perhaps assembled is a more accurate term. My light is build using a Solarforce P60 host (an L2M) and an International Outdoors P60 Lamp (a three mode warm).
This lamp does something around 400-500 lumens (honest lumens, not cheap Chinese lumens that are off by a factor of 5-10). I'd get the lamp in neutral or warm, not cool. Since I've bought they have added an LED with a CRI (color rendering quality) of 92, which sounds awesome. One problem with most LED lights is how "fake" they make everything look - that's basically because they are producing a very limited spectrum of colors. All these lamps are good, but the CRI of 92 is higher than I've ever seen before.
Here's a review of an older version of this same lamp module:
This flashlight will be limited to only 4.5 volts - so use either a single CR123 (possible in the L2m host) or a rechargeable 18650, which is my preferred option. Tenergy brand 18650s are the ones to get - they seem to be the best balance of cost and quality.