I wrote a post not too long ago about Firebug, Dragonfly, and the developer tools the web development community has grown to love and appreciate. Firebug, Dragonfly, Internet Explorer Developer Toolbar, and that thing on Safari all share one thing in common: they are extension of their corresponding browsers and when testing on a different browser it is often times hard to find the plugin needed, or tedious to get accustomed to a new interface. When things get dry in cross-browser bug fixing and DOM/CSS inspection, the new smaller kid in town is planning on saving the day - Firebug Lite. Unlike it's older brother, Firebug Lite is a chunk of JavaScript intended to be added to a web page that is being developed and provides an interface for debugging without the need of any extensions.
It's rather simple to use Firebug Lite. All you need to do is add in the script at http://getfirebug.com/releases/lite/1.2/firebug-lite-compressed.js.
There is also an offline method to use Firebug Lite as well as a bookmarklet for use during web development.
There are tons of other little features built into the tool, so go ahead and enjoy another method of debugging.