A Properly Calibrated Monitor
Last but certainly not least, your monitor is one of the most important tools you need as a landscape photographer and in order for the colors to look correct through various platforms, it must be properly calibrated! Otherwise, what looks good on your screen may not look good on other screens and definitely might not when you print and/or sell an image. You may be forced to re-process it and blindly attempt to make the colors look correct.
I use a Spyder4Elite from Datacolor to calibrate my monitors on a monthly basis (I calibrate my monitor, the laptop and my iPad when I had one).
The items listed above are those I find essential for my own photography and those I wouldn’t be without.Still, there are many more camera tools and accessories out there and I’m sure some of them are extremely useful as well.