Like most fieldherpers (or anybody that spends time in the outdoors for any other reason really), I have a selection of equipment that I carry with me into the field. Some I have with me at all times while some I only carry with me on certain occasions, depending on, for example, what animals I might encounter or how long I'm planning on being out.
My mandatory field equipment
There are a few things that I try and always carry with me into the field, regardless of what I can expect to encounter or how long I'm planning on being out.
Notebook - I make sure to keep a notebook with me at all times. For some species, I take notes on time, location, weather, altitude, moonphase, temperature, etc.
Pens - Multiple working pens. One will always die.
Field guide - Even if I'm herping locally where I feel confident about knowing the identity of any herp I may encounter, I carry one with me at all times. For the western US and Canada, I carry the current edition of the "Stebbins Guide" - A Field Guide to Western Reptiles and Amphibians - by Robert Stebbins and Peterson Field Guides. I feel it is the most thorough guide for this region and it's a great read.
Clean, dry socks - Although socks might not sound like an important herping tool, they are. There's almost nothing worse in the field than wet socks. They're also handy to have for first aid.
Food/water - I always carry at least a bottle of water. When I'm going on longer hikes, I've been known to carry up to a gallon of water with me. I know what it feels like to hike while dehydrated and what can come out of it, and it's just not worth it to me. I also try and carry a bag of jerky and some granola bars or something. Jerky has a good amount of protein and, well, just tastes good. I would recommend carrying non perishable foods though.
Small first aid kit - I always try and have a small first aid kit with me if I'm out hiking around anywhere. Bandaids, alcohol pads, Neosporin, tweezers, and moleskin will take care of just about any blister or wound you can get. If you're hiking any considerable distance, an ace type bandage never hurts either.
Nikon D7000 - I am currently shooting the Nikon D7000, however a lot of my pictures have been taken with a Nikon D80 and D300s as well. As far as the D7000, I absolutely love it.
Nikon Nikkor 105mm f/2.8 AF-S VR Micro lens - I used this lens teamed up with the D80 almost exclusively for the 2009/2010 amphibian season and it took some great pictures. It did take a while to get used to since I was use to an entirely different macro setup, which I actually prefer over this one.
Tamron AF 90mm f/2.8 macro lens - After being without one for a few years, I just picked up the newest version of this lens, the SP AF90mm F/2.8 Di, so most of the herp shots in 2012 will be from this lens.
Nikon SB700 Speedlight - I just made this purchase, but I was an SB800 user in the past, and I've heard nothing but awesome about this flash...
Nikon R1 Close Up kit - I've experimented with this in the past, but this will be my first season owning and using one a bunch. We'll see what happens!
These are some of the items I only carry with me on occasion.
Pillowcases - I'm not a collector, but I do carry a pillowcase or snake bag with me on most occasions. The main reason is in case I find something and want to hold on to for a little bit, say for a better photo opportunity. It's important that if you temporarily bag anything and shoot it somewhere else, make sure you always release it where it was originally found.
Non field equipment
There are some tools I use at home that help me out in the field.
Google Earth - Google Earth is one of the most valuable tools, in my opinion, that a herper can possess. I use it to find new places, keep track of populations I find, get info on altitude, latitude, and longitude for spots I have either found something or want to check out, etc.
I wouldn't have nearly as many spots or have found nearly as many animals without having satellite access of literally everywhere I could want to herp.