JASH : JavaScript Shell

From the website :

Jash is a DHTML-based window that gives you command-line JavaScript access to the current browser window. With this console you can quickly debug scripts, manipulate the DOM, view the current page’s objects, functions, and variables, trace the execution stack, execute arbitrary Javascript, enter new CSS (in IE, Firefox, Opera, and Safari), and much more.

You can simply add the bookmarklet to your browser’s bookmarks to use this tool, or if you want include this tool to your web page you can download them here.