If you don't have one of these little gems on your bench.....GET ONE!!!! My darling wife surprised me with one recently for my birthday.
What a nibbler tool looks like is an odd set of pliers. However, it is a fantastic tool for cutting square edges on styrene. I read about the nibbler tool in a model railroad book from the early 70's.
My wife found mine at Radio Shack. Unfortunately, for the Canadians on the forum Radio Shack has been taken over and no longer stocks these. However, you can still apparently find them from Radio Shack in the United States.
If you can not find one at Radio Shack check with www.micromark.com
I am currently using mine to make window and door openings for some background diorama buildings.
Follow this link and get one on your bench.
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=64-2960
Cheers;
Gregory