Jeff Atwood recently blogged about a nice feature of Vista, when you are entering your wireless network key. This feature provides the user with a checkbox to toggle the obfuscation of the wireless key being entered.
First, surround your password text field with a simple span
Add a checkbox to toggle the password obfuscation
Note I am using the handy $get which comes with ASP.NET AJAX. If you are not using this framework, you can just as easily replace $get with document.getElementById().
I have tested this in IE6, IE7, Firefox and Safari. It works as expected in all of these browsers.