Any unscrupulous person can find them out with little effort as this post by Roy Osherove explains.
With Firefox, the default behavior is to allow anyone to see your passwords as well, but you can actively choose to use a master password in firefox to make this less accessible. the default is that it’s open for anyone to see. With Chrome, there isn’t even a “master password” option.
IE, Opera, Safari store passwords if you ask them to remember but do not display them.
Also see:
IE8 Developer Tools Tips & Tricks