Energy Gels: I'm a big fan of Clif Gels when I need an energy pickup after a long hike or when I'm about to start one. If I'm tired and hungry, that can directly effect my motivation to use my camera if the conditions are less than ideal.
Cheap, Compact Umbrella: There's no way that I can keep my camera lens dry when a double rainbow appears during a downpour or while standing too close to an ocean blowhole without it. Along with the umbrella, I also keep a plentiful supply of dry paper towels in order to quickly absorb water and spray.
Footwear For Standing In Water: If I'm in Hawaii, this means my Teva sandals, but in southeastern Alaska, I'll be wearing my XTRATUFs. These brown rubber boots come up to my knees. You don't want to step in water deep enough to go over the top. Believe me, that's no fun. If I'm really going to be getting wet and crossing cold-water streams, I use my Patagonia chest waders and boots. I've abused these rugged waders from Alaska to South Georgia Island and have never gotten wet. I can't say that for my companions who didn't bring waders.
ROBIN BLACK (www.robinblackphotography.com) Tripod: Beyond the obvious mention of camera or lenses, which can and do change and evolve over time, what doesn't change is the following gear. A sturdy, dependable tripod is vital. Wind will kill sharpness in an image, especially with the heavier bodies and lenses most of us use. I use an Induro carbon-fiber tripod, and not only is it rock-solid for keeping my camera steady, it also survives a lot of abuse. I'm not gentle with mine. It gets banged against rocks and dragged through ice, mud and salt water—and this is generally true for anybody who shoots landscapes, so that investment is important.
Filters: I always keep a circular polarizer on my lenses when shooting outdoors, and for the last couple of years, I've come to depend almost entirely on the Singh-Ray Vari-N-Duo. It's a bit of an investment for a filter, but it combines their outstanding warming polarizer—which is ideal whether I'm shooting water or the red rock of Utah—and a variable ND filter with a range of 2 to 8 stops so I can get very creative with exposure. It does vignette at the super-wide range, but I can account for that bit of lost frame when composing my shot. The benefits far outweigh that one headache, and I almost never shoot without it.
Spare Cable Release: I learned this lesson the hard way a few years ago when I was setting up to do some "blue hour" shots of the Golden Gate Bridge and discovered that my cable release had fallen out of my camera pack at some point earlier in the afternoon. I was able to get the shots I wanted, mostly by using the timer settings on my camera, but it was beyond frustrating. I've kept a spare (or two) cable release in my pack ever since. They're inexpensive—about five bucks or so on Amazon or eBay—so just buy a few and keep them handy.
LED Flashlight: My keychain-sized LED flashlight has saved me more times than I can count—it's my third-level backup in case my headlamp or flashlight (or both) die on me in the dark. Just a week or two after purchasing my first one, I ended up using it on the hike down from Delicate Arch after sunset (not a stroll you want to take in the dark!). My headlamp battery died just steps down the trail when I started the hike out, and I'd left my flashlight in the car. The little LED light I'd tucked into a side pocket on my camera bag was all I had to see my way down the unmarked slickrock, and I'd have been lost—perhaps literally—without it.