I was working on a test site on WordPress.com yesterday and I noticed a Debug button in the admin bar. Clicked on it and some awesome info appeared that was extremely helpful with testing out the theme. Thankfully, the peeps at WP decided to release this tool as a plugin so everyone can take advantage of what it has to offer. Head on over to WordPress.org and download the Debug Bar plugin to try it out for yourself.
Adds a debug menu to the admin bar that shows query, cache, and other helpful debugging information.