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.
F12. (Oh, that's too short for an answer). The F12 key brings up built-in developer tools. It may not do everything that Firebug does, but it can do most things.
The F12-tool does the most things needed.
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".