Exploring the features of Firebug, with code examples covering using the console, timing code, stack traces, debugging and breakpoints, profiling, tracing HTTP requests. An excellent overview of Firebug's functionality
Using console.log outside of Firefox? Here's Joe Hewitt's simple JavaScript file to allow console.log to work on Internet Explorer, Opera and Safari. Reference the script in the page you want to debug, and add a class of debug into the HTML element, and you have a JavaScript console. There's also a command line - just like the real Firebug.
Webkit have released Web Inspector, which is a Firebug-type tool for Webkit and Safari. Contains Firebug related features like categorising resources, JavaScript console, HTTP network monitor, load time and resource size monitors, inline JavaScript and HTML error reporting.
Apart from being an elegant debugger, FireBug offers the developer hooks that can be used from JavaScript, offering logging, assertions, performance measuring, and command line functions for inspection or traversing a document.