Air chucks to pressurize cylinders through the spark plug hole. Essential when you need them, not used often.
Sockets to fit the top of strut and shock cylinder rods. Again, essential to take off rusted shocks/struts, not used often (usually use a crescent wrench) but when you have a stubborn one, really great to have.
Stud removers. For when pliers and vise grips aren't enough.
Left handed drill bits and splined broken bolt removers. Unfortunately, used more than I would like to.
I've got probably 100 tools like this that are low use, but essential. Hard to remember them, but great to have them when needed.