Here are the basics of what I take with me on every ride and why. Obviously it varies by the length, temperature, and type of ride but these basic tools will be with me 99.999999% of the time.
-Shimano chain pins / quick link
*on solo rides*
-MP3 player and headphones
Genuine Innovations - Second Wind mountain bike version
First things first lets take a look at the pump. Found this little gem probably about 5 years ago and I am lovin' it! So it's a mini pump, no big deal, gets the tires up to about 40psi with a billion strokes. The great thing about it however is that the head of the pump can be un-screwed and you can use it as a C02 inflator. It is an awesome inflator too.
Sooooo if you get a flat you can use it as a regular C02 inflator. Then if you get another flat you can use it as a pump when all of your CO2 is gone or you screwed it up the first try or you are too cheap to use a cartridge (me).
I replace this guy about every 2 years because like any CO2 inflator the seals tend to get a little worn out and it stops working effectively as an inflator. They make a road version that reaches higher pressure but I have yet to try that one. I think they are actually updating/upgrading this for next year so not sure if this specific model will be around much longer. You can bet I'll be one of the first to try out the new model.
Topeak Nexus multitool
This is what separates the mechanics from the guys who watched their buddy adjust their brakes that one time. This thing has everything you could practically want on a ride, meaning that you will be able to fix 99% of the trouble you might expect to realistically encounter while on the bike. Has all your common Allen key sizes, some spoke wrenches, couple screwdrivers, a Torx bit for disc rotor bolts, and a really great chain breaker (even has a little wire that holds the chain together while you repair it). There is even a slot that nicely fits a couple Shimano chain pins. I keep a 9 and 10 speed pin stowed in the rare occurrence of a Shimano chain breakage.
That's it. That is the stuff that is bulging in my pockets when I ride. Now you know.