Firefox: If you've ever sighed and said "Oh Firebug, if only you were wide screen!", today is your lucky day. Widerbug is a version of the popular web development tool tweaked for wide screens.
Firebug is a handy tool for examining the HTML and CSS code of a website along with the page itself. The original extension orients the site and the code horizontally. You can detach the panels and move them around, but then they don't update when you switch tabs.
Widebug is a tweaked version of Firebug, the site and the code panels are displayed side by side to take advantage of wide screen monitors. The panels maintain their connection to each tab, doing away with the phantom code effect found in Firebug when you try to coax it into wide screen mode. Widerbug is a free extension and works wherever Firefox does.