[dannyj621]

what are some tools you cannot brew without and you are thankful the day you bought it.  mine would have to be my sanitizer injecter

[dontblake]

The first thing that comes to mind is my siphon starter (the racking cane inside of a wine thief thingy). 
At first I thought that it was gimmicky, but I love how easy it makes transfers.  Not to mention sneaking samples of old mead.
Get one, it's worth every penny of the ~$12 it cost

Oh, and no homebrewery should be without a spray bottle full of star san.   That's probably about as essential as it gets.

[pinnah]

my undrilled bung.

No really, capping my carboy for manual labor aeration is critical to my methods.

[dhacker]

Stainless scrubbies. Get rid of those worthless green things!

[makemehoppy]

refractometer... no more waiting until just before I pitch the yeast to see how I did.

my other is 4 gallon plastic water bottles. I use them the store grain, others to hold brew water for a day or two to disapate chlorine, I use one for my gravity feed recirculate wort chiller*


* I don't have a good connection indoors for my wort chiller. I fill a bottling bucket with water and ice and gravity feed to the kettle that is on the floor. The output goes to a water bottle. The first 4 gallons of very hot water is saved for cleanup. From then on I recirculate (lift the output water bottle and refill the bottling bucket) until I get down to pitching temp.

[corkybstewart]

March pump
wort chiller
refractometer

[Beertracker]

My trusty refractometer, hydrometer & thermometer! 

[yaleterrace]

like a jerk, i didn't have a gravity sample thief, so i was sanitizing and using my auto-siphon to collect 9oz of green beer.  once i bought it, i never looked back!

[rabid_dingo]

I have to agree with pinnah. But mine is a threaded screw cap for the 6.5. Pick up that bad boy and like the song
says, Shake shake shake!

[Thirsty_Monk]

Refractometer is handy but I could brew without it if I had to.

I would say good brew kettle and wort chiller are essential.
And I guess some kind of fermentation vessel is also needed.

[ndcube]

My upgrade to a banjo burner plus windscreen from an SQ-14.

Also, my bucket that stays full of Star-San.

+1 on the SS scrubie.

[beerrat]

1) Immersion chiller (home made)
2) brew hauler (lessens back issues )
3) ball valve on my brew pot
4) wine thief
 

[tubercle]

Boil kettle and fermenter

[tygo]

Setting aside things without which you couldn't brew at all ...

I guess I'd go with autosiphon.  I don't use it on brew day but racking would be much more of a pain in the ass without it.

And actually as one who still has yet to experience the joy of kegging I'll throw in the $2.50 bottle filler. 

[tubercle]

Setting aside things without which you couldn't brew at all ...

 I'm sure that's directed at me.

 I change my choice to the trusty Barley Crusher.