The difference between using the right tool for a job and the wrong one can be the difference between a job done properly and one that's badly botched. These inexpensive tools are available at auto-supply stores.
Extra-long needle-nose pliers. Many know that needle-nose pliers are ideal for retrieving small springs, screws, nuts and bolts dropped in an area that's inaccessible to fingers. What many don't know is that you can get an extra-long version of the needle-nose pliers that lets you reach even deeper.
Crimping tool. This tool precisely removes the insulation from wires that are to be spliced or crimped together. Although a crimping tool isn't absolutely necessary, it makes the job quicker and easier.
Snap-ring pliers. These pliers are indispensable for removing and replacing snap rings that hold various shafts, gears and rods in place. Trying to remove a snap ring with a screwdriver or pliers often results in skinned knuckles and a lost or deformed snap ring.
Spark-plug terminal pliers. These are essential for removing spark-plug wires on many crowded engines where you can't reach the spark-plug wire boot by hand. The insulted pliers also let you remove the wire from the plug while the engine is running. Also useful for removing plug wires when the engine is hot.
Flexible ratchet. This is a regular ratchet except that is's hinged behind the ratcheting mechanism. This allows greater flexibility in removing hard-to-get-at fasteners. Especially useful when removing spark plugs from an engine crowded with a lot of accessories.
Screwdriver bits for ratchet. Available in both Phillips and standard versions. Also, Allen screwdrive bits.
Gear puller. A well-equippay mechanic may have several of these. The average Saturday mechanic needs only a general purpose two-jaw puller. Pullers are the only safe way to remove many pulleys and gears from their shafts.
Ring compressor. How do you put the pistons back in the cylinders after you've installed new rings? A hammer is not the way. You need a ring compressor to compress the rings then simply slide in the piston.