You can now generate promo codes for your Mac apps in iTunes Connect. These promo codes can be redeemed in any Mac App Store worldwide. For each version of your app, you can request up to 50 promo codes.
This is a bit of a turnaround for Apple after initially saying promo codes wouldn’t be available to developers wanting to offer free apps to reviewers and the like. Presumably this also means iTunes gift cards can now be redeemed via the Mac App Store too, whereas before users had to use iTunes to add credit to their account.