The
single most important filter for field work is a polarizing filter.
It removes glare which makes it possible to get good
pictures of objects with reflective surfaces like leaves.
It also makes it possible to take pictures of subjects that are slightly immersed
in clear water (tide pool), or behind glass (museum or aquarium).
It works best when the sun is directly behind you (opposite the
subject) but will produce an effect in most situations.
You simply rotate the filter until you get the best
effect.
A polarizer will increase color saturation and will allow you to
properly expose the sky and the foreground simultaneously.