First, an instrument with which to write.
Personally, I'm very fond of my Mac. It doesn't shut down in the middle of writing things, and asks very politely whether I'm interested in restarting now or later. I appreciate this. In the past I've mostly used Word, but for the current WIP I'm experimenting with Scrivener.
Coffee. Good coffee, freshly ground if possible. Dina will argue that tea is where it's at, but this is my post. So - COFFEE.
A cat. Yes, every writer must have a cat. I'm not entirely certain why, as the feline creature seems hell bent on making trouble for the already struggling writer. They sit on books and papers and laptops, knock things off shelves, and insist on being petted when hands are supposed to be deployed for writing.
Ice cream, brand of your choice. This is pretty self explanatory.
Whether for inspiration, celebration, or drowning your writer's woes, nothing beats a good bottle of wine.
Yep. Sometimes you run out of kleenex. Drying your tears (and your nose) on your sleeve is just not good. And there will inevitably be tears. There will also be moments of great joy and celebration. One of the great things about being a writer is that your readers will never know whether your tissues come from a box or a roll.
I probably ought to also mention friends. Because without them, no writer would have the courage and the faith to keep on writing.