I was looking around slashdot and found the following :-

Browse All You Want At Work
Posted by michael on Friday November 08, @03:48PM
from the corporate-drone dept.
choka writes "I came across a new Mozilla deriative known as Ghostzilla. It has the ability to open and hide the browser within most applications with simple mouse gestures, ensuring no one will discover what por^H^H^Hsites you visit in office (i.e., if your sysadmins don't check the proxy logs...)"
Ghostzilla Home Page

It would stop people looking over your shoulder, well maybe!