When shooting landscapes you will become aware of one simple fact; the sky is brighter than the land. Sounds reasonable, but this can be an issue.
A single shot with an unfiltered camera will produce an image with either a really dark foreground and a well exposed sky or a washed out sky and a well exposed foreground.
There are a few ways around this:
Multiple exposure shooting; this simply taking two or three shots at different exposures, these can then be merged together in post processing to produce an accurate representation of the desired shot.
Mid range exposure; this a single shot with neither aspect in an ideal exposure, the sky just to bright and the foreground just to dark. This can then be adjusted in post processing, by reducing the highlights in the sky and lifting the shadows in the foreground.
Circular Polarizing filter -
In street photography it will remove the reflections from windows and other reflective surfaces.
ND filters -
Graduated ND filters -