• Tough canvas carrying bag
• 6" safety wire twisters
• .032 safety wire (doubles as cotter key replacement in a pinch)
• Small nippers for cutting & stripping wires
• Duckbill pliers
• Vise-grips, full size
• Adjustable F-wrench
• Open ended long-pattern wrenches up to 1/2".  Make sure you have an 11/32" wrench, which is not commonly included in automotive-oriented wrench sets.
• Stubby open-ended wrenches up to 9/16", with the 3/4" or 7/8" also included depending on the spark plug wire ferrule size for the plane
• Spark plug socket
• Breaker bar of size for spark plug socket
• 1/4" drive short socket set up to 9/16"
• Stubby 1/4" ratchet driver and assorted extensions
• Cotter key removal tool - doubles as a small spike for hole alignment purposes
• Screwdriver assortment, ranging from small to roughly 1/4" slotted and Phillips screwdrivers, including stubbies and close-clearance ratcheting driver.  You could probably save space and weight by bringing an interchangeable tip screwdriver for all but the very small and stubby screwdrivers.
• Allen key set, I like the folding Swiss-army-knife style.
• Pickup magnet
• Inspection mirror
• Small flashlight
• Brass hammer (small)
• Utility knife with replaceable retractable blade
• Small hacksaw
• Straight metal cutting file, fine cut
• Round metal cutting file, fine cut
• One hemostat
• Spare spark plug and copper gasket
• Pair of leather gloves for hot spark plugs
• Several pairs of disposable rubber gloves in a baggie
• Several empty Ziploc baggies
• Small assortment of AMP wire splices
• Small assortment of trim screws
• Duct tape (very handy)
• Hand degreaser in a small plastic bottle
• Several terry cloth towels
• Small plastic container of multi-purpose grease.
• 1" screw-on air filler extension for filling tailwheels.  This small tool can be found at auto-parts stores and is a lifesaver since tailwheel air stems are often too close to the wheel to fit many filler chucks.
• 1 quart of aviation oil
• Two sheets of peel-and-stick Avery label paper, handy for writing logbook entries in the field that are later stuck into the actual logbook.

Depending on the airplane I may also bring a grease gun for the engine rocker boxes, a breaker bar for the propeller nut when flying my Cessna Airmaster, consumable fluids as required, and a bottle of light machine oil when flying Warner-powered planes in order to oil the magnetos every 20 hours.

This may seem like overkill but having a good set of tools has come in handy for me several times when I've been stuck in some podunk town out on the ramp, either working on my plane or a friend's plane.  Although you can usually find cheap tools in an autoparts store or a Wal-mart, first you have to get into town and then you end up with cheapo made-in-China stuff that rounds off bolts, strips screw heads, and often just breaks.  The more specialized aviation tools usually can't be found in auto stores so you're out of luck if you need them.