Let me start by saying, my bro-in-law calls me the "pack mule" whenever we go riding together because I bring so much cr@p in my camelbak.

For mt. bike rides, I bring in my Camelbak:

A spare tube
A patch kit
A folding allen wrech (2,3,4,5,6,&8)(I think!-Home Depot tool!)
A multi-tool (flat, phillips, knife, scissors, file, & pliers)
A chain tool (Park mini)
A spoke wrench (Work Force - 4 size)
A spare crank bolt
A couple of spare links and pins (taped over using clear packing tape)
A bunch of zip ties
A mini-pump

I also take some black duct tape and put about 5 or 6 wraps around my seatpost (the exposed part). It keeps it straight and is easy to get to if I need some in an emergency.
(Duct tape has a bunch of uses - but is great for a sidewall puncture, and for holding a bandage on for first aide!)

For my road rides:

1 spare tube
1 patch kit
1 mini-pump mounted below water bottle cage.
1 mini tool (Pedro's)
1 $5 bill.
4 quarters

I figure if my of my problems are flats, or bolts loosening up and the mini-tool fixes that.

The $5 is for emergency food intake (bonk induced!) (No Bokkie, not that type of Bonk!)

The quarters are for calling for a ride if anything else goes wrong!

L8R