In keeping with the original 'inexpensive' should have tag, how about:
PATIENCE....Aaargh..!! 

As far as must have tag, how about:
ACCESS TO THIS FORUM..!!

Motorcycle lift table - especially if your older and planning on playing  around on these.   Best investment I made.   Otherwise working on bikes for a long haul is painful on knees and back.

A nice organized workshop of reasonable size to work on bikes.   ==>  For the most part, I do it in my basement (walkout) but some bigger bikes (wider) I cant get through the door.   My second option is a pole barn which does not work good in minnesota winters - Not heated and barely enough power to run light bulbes
 

This is meant in the best  intentions. It's not a MUST have / or a SHOULD have

This is a THANK GOD WE HAVE


And that would be MARK T and the person I think as his double on the other side of the pond  Gr8uncleal

There are many others that keep the wheels turning also. I want to  personally thank everyone for making this forum what it is.

No I did not forget you MAKO 

You flatter me old chap - I still consider myself a mere novice! But many thanks anyway, the compliment is much appreciated. 

1. Mako, MarkT, Enduronut, Gr8tuncleal.
2. This thread to get new members started
3. Factory service manuals and parts lists. (I like originals or copies of)

A 5 gal bucket of Evaporust. I think I've submerged 5or6 rusty tanks in the same bucket and countless parts. Also a set of JIS screwdrivers

YAMABOND

Seal Drivers

A very simple thing that has taken me years to do as i'm obsessive.
1.The ability to say that's enough for today, go and have a brew and think about what's next so you don't fook the job up.
2.Don't give up, you have the ability, just have faith in your ability.
3.Love what you do. When it's a chore and you don't want to do something then the inner eye isn't there and that little tingle you get where you " just want to stay there" can't be felt. 
4.If you do it wrong, don't be defeated. Let the steam off and calm it. Go back and do it again.
When you stand back and the inner Dave says to me " Fuck Dave ,you've done all that ! "it feels brill.
So the best tool i suppose is YOU !