This is a picture of a new, modern mini tool for carrying on a road or mountain bike, and below is a photo of mine after many years of use.
So, it's obviously a well used tool! Here is the list of 'must haves' to carry on every ride:
1. Allen keys
4, 5, and 6mm are the main ones; these three alone will adjust 95% of bolts on the average bike. I also like to carry a tool that can handle 8mm crank bolts (usually via an adapter that slips over the end of the 5mm). Most tools also cover the smaller sizes for micro adjustments like on road brake callipers and mountain bike disc pads.
2. Flat and Phillips head screwdrivers
Handy for adjusting derailleur high/low limit screws, and also various bike accessories that still use bolts with a screw head...must have tools.
3. Tire lever
Obviously to change a tube when you get a flat. These are often fitted to the tool in ingenious ways to keep it compact. It has been commented before that you don't need tire levers, "just use your hands", but why risk breaking a nail over the sake of a punctured tube? Tire levers make the job sooo much easier and protect the manicure at the same time.
4. Chain tool
A must have as well. While it is rare to break a chain, when you do it is a matter of fixing it or walking home. I hate walking when there is a bike; save your frustration and carry a chain tool (this is particularly applicable to MTB riders out there).
5. Spoke key
Good to carry, better if you know how to use it, especially if you use rim brakes!
This razor sharp 'hunter's knife in miniature' probably won't be any use cutting out a deer's heart but you might be able to use it for small game; like lizards or field mice. Perfect for the survivalist mountain biker and "Bear Grylls" wannabe.
7. Bottle opener
How else to open the post-ride beer?
8. T25 torx (but not on my tool as I don't need it. Read-on)
You may only use it once or twice, but for those couple of times it will be a day saver. Used to tighten disc brake rotors (unless you use centrelock hubs, which you should like me, because they are better....then you don't need a T25).
Any other suggestions? I can't really believe I could have forgotten anything, but I have been wrong before....only once or twice....and i'm still not convinced it was my fault....