In reply to DancingOnRock:
Mountain biking:
Topeak Mini 18+ multi tool
Blackburn pump
Leatherman tool.
Spare rear mech hanger
A few assorted zip ties
10mm combi spanner if on old bike.
Schraeder valve tool for fork valves, after I once had a leaky valve.
Depending on who I'm out with, I've also taken a small adjustable spanner, decent tyre levers, chain lube, a few spare bolts and spare brake and gear inner cables - these have got me/us out of a few holes.
Road:
4 and 5mm Hex keys
Maybe a chain tool - Park mini chain brute.
CO2 pump + spare gas. On really long/remote rides, I'll take a normal pump.
Both:
Tyre boot (toothpaste tube)
Spare tube
Puncture kit
Chain quick link
Usually latex gloves for messing around with the chain.