Not many have the capability that I know of but they do exist.
The French Navy Alouette 11's I saw at Frejus Naval base in the Var region of France in 1988 had the capability.
An example for use - if you were searching for survivors and found them before you had burned off much fuel you could dump fuel to allow you to hoist the survivors aboard.
Alouette II
Only other helicopter I ever saw that could dump fuel was a Court helicopters S-61 in Cape Town SA. which had a belly mounted aux tank that could be jettisoned completely.
Example of use - coming off an offshore helideck - lose a stove - one click - you are 1500 lbs + lighter ( I don't remember the combined numbers for weight of tank and fuel) but it was a big tank and you would be in a much better position to fly away VS ditching the beast in the cold South Atlantic.
As I recall they used to use it to do crew changes on ships transiting around the Cape of Good Hope and they were sometimes a long way offshore.
Only two I ever saw but there must be more.
I am sure someone will be along shortly with better information.
You would probably get better results posting this question on the rotorheads thread.