For
free, costs
nothing,
bugger all, a
freebie.
Very occasionally heard in public houses, when very nice landlords decide they are going to bequeath a punter with a free beer, either for a particular reason (birthday, engagement, blue moon tonight, hell freezing over) or just because the barkeep is feeling generous (equally rare).
Comes from a shorthand way of saying "The house is paying for this one"