One of the essential tools I have in the field (although admittedly usually on my person, not in my bag) is a knife. This gets used pretty much every time I go in the field. I often find myself cutting line, bait, or kelp, prying samples off of rocks, removing cactus from my boots (or legs), and prepping lunch. Given where and how I work, this has led to a pretty rigid set of requirements:

Small and light, with a sturdy clip
Saltwater resistant
One-handed open and close (either hand)
Lock-back
Good grip
Not a clip, spear, or drop point (in other words, not a super pointy point)

It wasn't until about 8 years ago I found what I considered a great field knife. It was a Cold Steel Voyager, with a tanto blade. While it was never billed as corrosion resistant, any rust comes off easily with a kitchen scrubber, and the tanto blade (see that nice flat edge at the end?) allows for it to be slid under stubborn chitons or limpets to pry them off rocks.

The problem is, Cold Steel changed their design a few years ago (2011 I think), and while you can still get one with the same length blade, it is now wider, with a bigger, heavier handle. Still a good field knife, with an admittedly better grip to it, but that extra heft is more than I want or need.

Thankfully, I found another field knife that I also love. It is a Spyderco Saver Salt. Not quite as small as my old Cold Steel, but still light, with H1 steel that has incredible corrosion resistance, and a ~3" sheepfoot blade that makes it safer on rough seas and slippery rocks. Better still, it comes in yellow, which I like because it makes it easier to keep track of and also seem less threatening (handy when at border crossings, for example).

My beloved and well-worn field knives - older model Cold Steel medium Voyager on top, newer model in the middle, and Spyderco Saver Salt on the bottom.