[Rusty Jefferson]

1- A Stereophile test disc with left/right, phase inversion, and 1khz test tone for level matching. I can't remember how many times I've arrived at someone's house to listen and there has been a left/right reversal, or actually speakers out of phase and nobody noticed. And then wanting to compare equipment and trying to level match with an SPL meter. You must match voltage which means....

2- A multimeter to match voltage. Doesn't need to be expensive.

3- Headlamp like for camping.

4- 2.0+ reading glasses even if you don't need them because it makes it easier to see things up close.

[Early B.]

The most basic thing you need before building almost anything is a plan. Build your audio system around one or two things you like most in music (regardless of the type of music you listen to!). For instance, if you love bass, then build a system around getting the best subwoofers, researching bass traps, eliminating the 50Hz bass bump in your room, etc. You'll take on a different approach if you're a treble guy or a silky midrange guy or the tube guy, or the heavy metal & hard rock guy. 

Example -- I wanted a big, dynamic sound, so I have 3 subs and high-efficiency speakers with 12" midrange drivers. I wasn't gonna get a big sound with 6" drivers, regardless of the brand or style, so I stopped wasting time and money on "small" speakers.   

IMPORTANT -- do not include a budget in your plan. Most audiophiles spend more money than they want to, so don't impose a money barrier. Hobbies aren't meant to be rational.
 

[Bhu108]

Green pen (like a sharpie) for the edges of CD's
quality masking tape to mark speaker placement
Blue Tack, earthquake putty, or similar for vibration damping etc
screw drivers
microfiber cleaning cloth

[bgorman]

An abundance of conformational bias

[Aspirant Audiophile]

An older, obsolete, cheaper system. I get a lot of understanding not to mention enjoyment out of comparing what I have with what I had and what I want... Usually the differences aren't as vast as I imagine

[AllanS]

The most basic thing you need before building almost anything is a plan.
 

This

[charmerci]

An abundance of conformational bias

I'm cheap as shit. I don't want to spend tons of money on stereo. I don't want my ears to hurt (trust me when the kids at work turn up the lousy stereo, it emotionally and psychologically HURTS and I want to quit). I would LOVE IT if a $50 IC would not be harsh compared to the very expensive Hapa ICs, when I crank the music. I SO WISH that the much more cheaper, same gauge speaker from the hardware store sounded as smooth as the much more expensive GR Research speaker wire. Believe me, for 30 years I thought via Audio by Van Alstine philosophy that all wires sound the same until surprisingly, I noticed that they didn't. (12 years ago.) And how did I notice? My body/brain didn't quite vicerally react to harshness of the sounds emanating from my speakers when I turned the stereo up. I would LOVE more than just about anything in my life to spend $500 on a complete stereo that I could listen to loud without it hurting, literally.

It doesn't work that way. And a bunch of incredibly intelligent (and successful) people who make it their life's work also think so.

Also, if you believe that, it can work the other way too. If you believe that it doesn't make a difference, you WON'T hear any difference.

[Zuman]

I've just spent a lot of money on the AnalogMagik V2 cartridge alignment system and test LPs (it hasn't arrived yet). I really didn't want to do it, but it is absolutely indisputable that I (and others) can hear significant differences resulting from changes of less than one mm in tonearm pivot height in my system, and it's clear that other cartridge alignment tweaks also change the sound from my Clearaudio Performance DC AiR with Tracer arm and Hana ML cart. I've fiddled around with protractors and acrylic blocks and bubble levels enough; I want to get it right, and be able to quantify what change results in what outcome.
When "serious audiophools" spend thousands on cables, tube-rolling, power conditioning, racks and vibration-control, room response management, and so many other variables, I finally decided that I'd try to nail my analog front-end once and for all.

[edpetk]

Surprised and happy to see someone else put the Measuring Tape right up front! Re hex tools, I keep the super tiny one to remove my integrated's cover masking-taped to the side of the unit. And I still sometimes forget where it is.

[jimdow]

A shrink

[rotarius]

[bimmerlover]

I like to use a laser "tape" measure. It's a lot easier to use alone and for longer distances, where a tape measure just collapses or is hard to handle.