Through Parallels, Mac users can run Windows programs. Regardless of what operating system you run, as a web developer you should never ignore Internet Explorer. There are still plenty of people using the browsers because it still runs natively in Windows.
The best way I found to get IE6 and IE7 on my Mac was to use parallels and upgrade to Windows to IE7, then install MultiIE to get IE6 running. Take a look at this screenshot to see it in action:
It looks kind of strange to see a windows taskbar on a Mac, but hey – now I have both IE6 and IE7 running great on my Mac to fulfill my web development needs.