Tunebite appears to be a way to break DRM off media files. More info would be needed on what exactly Doug uses it for to find an alternative.
ZDoom is just a windows specific version of the Doom game. Plenty of ways to play Doom on pretty much any OS or device these days since the code has long been GPL.

And pretty much nothing in that list prevents a migration to Mac OS X either.

I've seen a lot of people switch to OS X recently from Windows, and their first comments do tend to be centered around "Application X doesn't exist for OS X". Breaking down what they used an application for usually results in finding an equally capable alternative. And in the end, switching between one OS and another is usually no less or more painful then switching between Windows XP and Windows Vista.