Most Useful for me have been:
A Drill--Just makes my day easier for milling grain, and for aerating the wort.
Refrigerator--Have a place to store hops makes my kitchen refrigerator much more useful
A Large Plastic Box--Cheap and useful for fermentation temp control. In my current house I can get the temp down into the low 50's, and made some very good lagers this Winter.
Vacuum Sealer--Extends the life of my hops for a very long time. So, no worries about buying in bulk.
Hydrometer--I still use it for every brew, but I have my process down pretty well at this point, and am never surprised. So, I think I could very well stop using it, and know what the numbers are for my brews.
Not too much else I find to be of little use. I'm not very fancy in my brewery, so pretty much everything has a good use.