Firebug

There’s now a version of Firebug that runs in IE, Safari, and Opera. It works as a bookmarklet or a javascript file. The profiling, debugging, and network sniffing features that make the full version of Firebug invaluable in Firefox aren’t there, but the DOM browsing feature alone makes the bookmarklet worth it.

Opera Dragonfly and Safari Web Inspector are the more full-featured, browser-specific alternatives.