Javascript debugger for IE 7

I've been trying to get some custom extentions to the FCK editor working and I've been having problems on IE. The problem is that things just act goofy on IE and I don't have any good tools to debug it. Is there something like firebug or a good (free) javascript debugger for IE 7, if it were to work with IE 6 that would be a plus, but even just 7 would help.