What is this plant with purple berries?
It looks to me like beautyberry. Here is a link to a species page to judge for yourself. It's a very common cultivar and I've heard of people using it to control garden pests. 

http://uswildflowers.com/detail.php?SName=Callicarpa%20americana

Here is an image of American Beautyberry from gardenknowhow. 


It is often confused with Japanese Beautyberry which has smaller berries on stalks.