My flashlight. This isn't something we think of as a brew house tool, but I can't say how often in a brew day I'll need to randomly retrieve old Flashy for one reason or another. If this was a Western, Flashy would be my horse. I don't know if I could brew without him.
Most importantly, I use Flashy to inspect the cleanliness of kegs and carboys (you would be surprised how many particulates show up under direct light that you can't see in room-light) but I also use my trusty flashlight to find other tools, to check the clarity of wort, to peer through carboys, look into dark fermentation chambers, evaluate steam and boil off, and to highlight things for brew day pictures. During my indoor brew session when the power went off, you can imagine the first thing I did. Ok actually the first thing I did was grab a bunch of beers out of the fridge while they were still cold, but the second thing I did was grab flashy, lit the gas stove, and got back to brewing. I imagine (like the flashlight pictured up top from Silent Hill) he would also do double duty during a post zombie-uprising brew day as well. Anyone else have any good under appreciated/unexpected brew house tools you can't brew without?