In addtion of window.js, there is a little javascript debug.js more usefull than the alert javascript function to display debug information. There are few functions:

• showDebug() : to open the debug window.
• hideDebug() to remove it.
• debug(message, reverse): to display a message in the debug window. reverse is optional if you want to append text at the beginning of the window instead of the end.
• clearDebug(): to clear debug window content.

Just add this lines in your HTML page <script type="text/javascript" src="/javascripts/prototype.js"> </script> <script type="text/javascript" src="/javascripts/window.js"> </script> <script type="text/javascript" src="/javascripts/debug.js"> </script> <link href="/stylesheets/themes/default.css" rel="stylesheet" type="text/css"></link> <link href="/stylesheets/themes/debug.css" rel="stylesheet" type="text/css"></link>
And this in your code <script>showDebug()</script>
This window uses setCookie() to save its size and position. You will see after using it you cannot live without :).
Click here to open the debug window that displays mouse coordinates.

Add on

Nick Hemsley has done a nice add-on to the debug window that allows you to inspect any object.
Click here to inspect the H2 "Add on" title.

Add on - extended

Jason Pollard has included the Nick's add-on inside the window debug by adding a "inspect" icon in the top bar.
Click on the icon and then click on an element in the page you want to inspect.
He has also included an eval text field with history (use up and down keys).

To use it just include extended_debug.js after debug.js.
Click here to open the extended debug window.