For a little over a year I've been working with Microsoft on the modern.IE site. This week the site is celebrating having saved developers one million hours in browser testing. Some of the savings come from the free pre-configured VMs with real copies of the different IE browser versions. Just about all the popular VM software is supported on several different platforms. There is no substitute for using the real browser when it comes to compatibility testing, and these VMs are one way to do just that with minimal hassle.
Further time saving comes from Browserstack. It provides convenient in-browser testing of just about any type of browser or device. Modern.IE offers a free three-month membership to Browserstack for IE versions. There are Chrome and Firefox plugins that make it really easy to quickly try out your latest designs on any version of IE, even if they're served off your local computer. Very slick.
The scanner project is open source, written in node.js, and hosted on GitHub. This tool is meant to be useful to your web development diagnosis and debugging, so open an issue in the repo if you find problems or have suggestions.
For even more information, you can run your site through Compat Inspector via Sauce Labs to find code specific to older versions of IE that might cause things to break with modern IE versions. To use this feature, just click the "Scan for code no longer supported in modern IE" tab once the initial scan is done. On that same results page you can also get screen shots of how the page appears in various browsers, as another quick check that the layout is functioning properly.
Quite often I'll hear developers say, "All these versions of IE are a pain and make my job harder! Why are there so many versions and why can't I just run this on a Mac where I do my development?" There are so many versions of browsers, not just IE, and yes that is the burden of the web developer. The Android 2.3 browser is just as much of a pain to today's web developers as older versions of IE, and that will get worse before it gets better. New phones are still shipping with that ancient Android browser, but at least the market share of older IE is shrinking.
As for why you can't run IE on your Mac, let's turn that around and ask why Windows-based web developers can't run Safari to do testing. Apple doesn't make VMs available to do that the way Microsoft does here. Fortunately, Browserstack does support many versions of Safari. Perhaps Apple will make free Browserstack testing available for Safari. No matter what, though, responsible web developers should test on every browser they expect the audience to use. Microsoft has done what they can to make that easy.