My name is Maarten Metz and my initials combined with the last letter of my last name pretty much explain the 'MMz' part of this domain name. But what's up with the Mx?
Since I'm a software engineer, most of my time I'm reading, writing and changing text on a screen. The emacs editor is my main tool to work with text. This editor is highly customizable and has a lot of functionality. Emacs users often use keyboard shortcuts to 'invoke' that functionality.
If you don't remember the keyboard shortcut for a function (or the function has no keyboard shortcut), you can always invoke it by guessing its name. That's done with the M-x command. So for instance you can call 'M-x goto-line' to jump to a certain line number, or 'M-x quick-calc' to invoke a simple calculator.
Every function in emacs basically looks at 'the world' from a very specific perspective. Therefore when people type MxMMz in their browser bar I like to think that they invoke the MMz perspective.
Now that is cleared up, you should really hover over the 'MxMMz' text in the header of the page to discover a small easter egg that was there all along. And don't forget to M-x MMz every once in a while!