The Northwest late autumn and early winter is wet and cold.  This is my currently indespensible item list:

1. A dehumidifier in the tackroom. I think it speaks for itself. 
2. A laundry line and/or lots of hooks in the tack room so wet but clean items can actually dry.
3. A water heater. My barn is not plumbed so I use a portable propane water heater.  It doubles as the camp heater at shows.
4. The old fashioned mesh sweat sheets under a sheet.  It just works better  than fleece or waffle weave when the air is so wet and you need to dry off your horse.
5. Rubber palmed work gloves.
6. Towel bath items.  Lots of towels - small for washing and big for rubbing dry, preferred shampoo, some spot remover like Cowboy Magic, and baby oil.
7.  A wisp.  Okay, I was raised in Pony Club so I call it a wisp but I use a loofah or a sisal grooming mitt.  But its great for finishing grooming or rubbing down between blankets and stimulates oil in the coat.
8. Electrolytes.  Several of my horses don't drink enough in the winter, so to make sure they are getting what they need, a little Apple a Day in the water makes me feel better.
9. Ahhh. Toe warmers. The squeeze kind you get for hiking or camping.
10. Synthetic tack.  There, I said it.  I have Wintec.  If I am riding 3-5 days a week in pouring rain with no indoor arena, I am not going out there in leather.
11. An old fashioned real wool quarter sheet.  Fleece gets soaked.  The waterproof kind will rip and make scary noises.  Wool can get wet and still keep your horse warm.  I found mine on ebay as I couldn't find one at tack stores.
12.  Waterproof turn out sheets and a rain sheet.  I hate to turn out horses in blankets but I hate wet muddy ponies at riding time even more.  I also have light rain sheets to throw over horse or tack to get from A to B for schooling shows or a lesson or in the trailer. The rain shets are too flimsy for turn out, so don't even try.
13.  Stall entertainment.  More time in the stall in winter means keeping those wild ponies busy instead of bored.  Their hay is always fed out of a nibble bag to make it last.  They enjoy the barn cats visiting and music playing.  Otherwise I rotate balls, hanging toys, large dog toys, treat dispensers, stuffed animals, whatever they express an interest in they get, within reason.  I know many people use mirrors, but my horses are freaked out by the intruder in the mirror.
14.  A vacuumand/or body clippers.  If you don't clip, sometimes the vacuum is the only way to get them clean without a full bath.  I've used expesnive equine vacs and now I use a small, portable, dedicated (to horses bodies only) wet/dry vac.  It does the job just as well.  The ones I have had for 3 years I got on sale for under $40.  Otherwise, Clipmasters are king.
15.  Freshened kits and emergency supplies.  Season changes are my cue to check first aid kits, emergency kits, evac supplies, etc.   We flood, freeze and lose power in this season so having ready extra feed and hay, back up power, light, de-icer, etc can be critical.