Carpenter’s Pencil, don’t be caught without one
Having bought all the key hand tools you need, there is something that isn't a hand tool but that you just couldn't do without, in fact, you couldn't even start your project.
Pencils. Lots of them. No matter what project you are undertaking you will constantly need to make a note of something: mark the spot that a length of wood should be cut, place an 'X' on the wall where a light fitting would hang, and, at the end of the day make a note of the supplies you need for the next day.
You will most likely think that you can get by with any old pencil, even take one from your kid's school supplies. Unfortunately they are just not sturdy enough to go through the rigors of D.I.Y projects and live to face a second day. If you drop it you will find that the lead breaks into many pieces, but you don't know until you try to sharpen it – the lead just falls out at the break line.
If you are shopping at your local small hardware store you will often get a carpenter's pencil free. Failing that, though they do cost about 50cents each, they also last a long time. Pick up a handful, and get the colored ones not the ones in unpainted wood. When you are looking for one, it is so much easier to find a bright red or yellow pencil than it is to find one that is the same color as the wood we are working with.