To see this tool in action, visit http://krofdrakula.github.com/microdata-tool.
This tool was written to aid in debugging and browsing HTML5 microdata.
To use this tool, simply include the script anywere after the inclusion of the jQuery library. When you view the page, a list will appear in the bottom left listing all the microdata objects detected on the page.
In addition, you need to provide the schemas to validate against; some are already
implemented in schemas.js which is to be included immediately after the
jquery.microdata.js script. To add your own, you can use the
$.microdata.addDefinition(url, fields) function as used in schemas.js.
This tool is a work-in-progress and still doesn't adhere to the full specs (yet). Plans to support the full microdata spec + data vocabulary validation are on the drawing board. ;)
Thanks to @peterhost we now have a bookmarklet available (source: https://gist.github.com/1397528) for your use without having to modify your page's source code! Simply visit this page and drag the link to your bookmark bar and you're set to go.