If anyone saw the movie "A Serious Man", you may remember that the protagonist's employer received anonymous poison pen letters regarding himself. I know someone that happened to in real life. His name is Louis Proyect. He was a student in the 1960s and got involved with the anti-war movement, and then became a programmer at Metropolitan Life still active in the anti-war movement. The FBI used to send anonymous poison pen letters to his employer, Met Life, in order to try to get him fired.

Actually he got his hands on some FOIA documents after a lawsuit, he has some of them on his web page:

http://louisproyect.org/2007/08/19/encounters-with-the-fbi