A good hammer and saw, lots of lumber, nails, concrete and shingles- garages ALWAYS need expanding!
On a slightly more serious note, my essentials are:
- good sets of SAE and foreign wrenches, including the stubby ones;
- good sets of SAE and foreign sockets, along with a quality fine-toothed ratchet;
- quality torque wrenches, a foot-pounder and an inch-pounder;
- an air compressor;
- oil filter wrenches; and
- a good floor jack.
If I typed out everything I have in my garage I would be banned for crashing the entire Internet!