I know that high wingers have an inherent dihedral effect.
Are there any examples of high wing aircrafts with a dihedral? If so, what is the reason behind it?
High wing planes typically have less dihedral than an otherwise comparable low wing design, because of keel effect: they have more righting moment for a given dihedral angle because the fuselage hanging below the wing increases pressure near the wing root on the low side when a bank induces slip.
The value of the dihedral can be low enough that the wing may look flat (Piper Cubs and Taylorcraft look flat, to me), but if you build a high wing airplane with genuinely zero dihedral, the wing will appear to droop (take a look at period photos of Lindbergh's Spirit of Saint Louis).
Dihedral is present in nearly all high wing aircraft, for the same reason it's present in nearly all low wingers -- because it tends to support or raise the low wing when a bank without coordinated rudder causes side slip. This produces lateral stability, which in turn makes the airplane easier to fly.
Depends mainly on the shape and size of the fuselage, and if the plane has wing sweep or not.
The high wing aeroplane has a wing/fuselage interference effect that tends to roll the aeroplane away from the sideslip direction, the low wing aeroplane wants to roll into the sideslip direction.
The second contributor to roll moment from side slip angle is from swept wings. The figures above are from my treasured copy of prof. Gerlach's uni book, the second one shows the difference in air speed over the wings, and therefore the difference in lift causing a stabilising rolling moment, away from the side slip.
Dihedral (positive V-shape) provides a positive roll-sideslip stability, required on low wing aeroplanes. With high wingers, at larger fuselage diameters the stabilising effect from interference and wing sweep can be excessively high, and then anhedral must be applied.
Other high-wingers do benefit from dihedral.
Yes, many if not most high wing aircraft (general aviation at least) have dihedral. The reason is the same as for low wing aircraft--to improve stability. I had a remote controlled high wing slope soarer glider that I built which had 0 dihedral. It was very maneuverable, wings tended to stay banked, unlike other aircraft with more dihedral that have stronger tendency to level out.
The dihedral on a high wing aircraft, be it positive, none, or negative (as illustrated below) will depend on the engineering and performance goals of the aircraft being designed. If you're building a primary trainer its going to have a lot of dihedral, even if its a high wing. If you want maneuverability then it will have less. In the case of the Harrier below, I imagine there were performance and engineering reasons for the extreme anhedral.