in roman catholic churches, what is the tool that priests use on Easter to sprinkle holy water on the people called?
The tool used to sprinkle the holy water is known as aspergillum. It often comes in two forms: a perforated ball or a brush that is dipped in water and shaken. For more information, go to http://en.wikipedia.org/wiki/Aspergillum.
It is called an aspergill or aspergillum.
Just be careful because holywater is not all that "holy" because of the amount of people that put their fingers in it as they walk out the door using it to cross themselves.