I've got several bags that I might use and I'd always forget things if I had to pack them for each ride, so I keep the essentials in each bag: a multitool, tire levers, a pump, a patch kit and a spare tube.

The bag that I usually use for commuting has a good bit of storage so it also contains a small bottle of chain lube, some spare rim tape (someone here suggested it as an emergency repair for torn sidewalls), a small first aid kit, a few spare chain links and back-up lights. The multitool in that bag is a Crank Brothers multi-19, which includes a chain tool. Some of my other bags have very minimal multitools.

When I use my road bike I often gamble with just a small seat bag that has CO2 cartridges instead of a pump, though it bears emphasizing that this is a gamble.