Is there something like Firebug for Internet Explorer? I've got a site that is only supposed to be working in IE so I'm only targeting IE and the site is not supposed to work in Firefox.
Chrome and FireFox have generally been the ones with the best expansion capability and built-in and third-party developer tools. However, IE9 has built-in developer tools allowing you to browse and step through client-side code in much the same way as Chrome's developer kit or FireBug. It's accessed, as paulmorriss said, using the F12 key or by opening the menu (the gear icon on the far right) and selecting "F12 developer tools".