Sometimes you discover a plugin and wonder how you ever survived without it.
For me, that's true for the "What Template Am I Using?" plugin. This is like Firebug, but specifically for your WordPress site. It's incredibly useful for theme developers. The plugin shows you the current template file you're using, plus any additional files being called such as header.php or footer.php. Oh, and it shows lots of additional information as well.
In this video from class "Introduction to WordPress Theme Development", Topher introduces you to the WordPress theme hierarchy and to this very useful plugin.