I think everyone should carry some kind of knife and light at all times. Given how little effort and expense is involved, there are just too many times when they are really useful not to keep them with you. Less common, but more important, are the occasions when they can make a critical difference.
A first aid kit may not be something you need to keep in your pocket all the time, but it is worth tucking into a purse, backpack, briefcase, or glove compartment so you'll have it nearby when you need it.
Survival gear is definitely not an everyday item for most people. However, a basic kit is a very good investment when travelling or spending time away from urban areas. You never know when something could go wrong.
A knife may be the single most useful item you can carry, both for everyday use and for all kinds of emergency situations. The Utili-Key is not much of a knife, but it does provide a small blade and a minimal set of tools in a package the size and weight of a normal key. Add it to your keychain. Remember that the most useful emergency item is the one you have with you when you need it.
I strongly recommend carrying a larger knife or multi-tool of some sort with the Utili-Key as your emergency backup. If you don't regularly carry a knife take a look at my knife and tool collection for some ideas. They come in all shapes and sizes; there's sure to be one that will suit you.
When travelling by air: Remember to pack any knives in your checked bags.
A light can be useful both for illuminating your surroundings and for signalling . The Photon II is tiny, lightweight, tough, and long-lasting. Put one on your keychain and you won't notice it's there until you need it.
This is a very basic set of first aid supplies in a small waterproof pouch. It is handy for casual use as well as for emergencies. The kit will fit neatly in a back pocket or comparable space.
In addition to a generic first aid kit, make sure you carry an extra supply of any medications you normally take!
When travelling by air: Remember to remove the knife, safety pins, and any other sharps and pack them in your checked luggage.
This is a basic outdoor survival kit in a small waterproof pouch. It is a little thick for regular pants pockets, but will fit in a cargo pocket, coat pocket, belt pouch, or purse.
The kit includes items for lighting fires, signaling, improvising shelter, navigating, and other survival necessities. This will not allow you to surive in the wilderness for months, but it can save your life if you run into unexpected weather, mechanical problems, or other hazards.
When travelling by air: There are several things in this kit that will probably not make it past security. Remember to pack it in your checked luggage.
There are a lot of other things that could be useful in an emergency, but I think this represents a good compromise of utility vs ease-of-carry. These items are small and light enough that most people will be able to carry them without noticing the extra load. Start with this list and add whatever your personal situation suggests might be important. Just keep in mind that having the perfect solution to a problem…at home…will not help you in a real emergency.