So not specifically automotive, but useful for EVERYTHING. I know, not quite the purpose of this thread, but who said 'multi-tools' don't work? (all Harbor Freight, but hey)
This:
Hex Drill Socket Driver Set 3 Pc
Turns any drill into ratchet.
and this:
1/2 in. Heavy Duty D-Handle Variable Speed Reversible Drill
The torque on this thing will near sprain your wrist just pulling the trigger 'dry'. Good for drilling out rusty screws and bolts, wire brushes, buffing pads, stirring paint, etc. Super effing heavy duty. For $42. I've had mine for 3 years and worked it for 20+ minutes straight at times and it's still just as powerful.
Also, a good clampable workbench like this:
Folding Clamping Workbench with Movable Pegs
Also a can of WD-40. Most every screw, bolt, hose, clamp, etc, gets a spritz before working on it.
Also makes SPECTACULAR hand cleaner. A small spritz in the palm, lather up, rinse off with the water hose, and then shop towel dry. A hell of a lot better than any soap I've ever used. Even cleans under the nails well.
Make sure to use the HF 20% off coupon, and get something free with each purchase as well. I usually get the multimeter or 6 piece magnetic screwdriver set. I have maybe 5 of each now.