Caldera Cone – It's just a genius design with the pot stand being the wind shield. Very stable, very efficient and cool looking too. I have three of these now: the Keg-H, the standard cone for the SP900 and a ti-tri with inferno for the MSR Titan.
Buff – it's always in my bag/pack, being used year round as a cap, scarf, mask etc. Everyone should have one. I've recently bought the Polar Buff too, which I like.
Lundhags Traverse Pant with Schoeller strech material. Very comfortable! Only downside is that it is a cotton-polyester lend pant, so will wet through eventually. A bit heavy too, but worth it.
Honorable mentions:
Trekking poles (Leki) – just makes it easier, faster and more fun to hike. Great multi-use item when used as poles for a tarp too.
Swedish fire steel with quick-fire tinder
MSR Titan kettle
Jason Klass's lid for the SP900