Stuff My Stocking by M/M Romance GoodReads' authors
Normally with an anthology I like to break down the individual stories, but there are too many to do that here, plus I read them over a long period of time (6 months). A good chunk of these stories (if not a majority) are written by well-known authors in the genre, so unlike some other free anthologies (assuming there are some, somewhere, aside from Wishing on a Blue Star), the majority of these stories are well written, well edited and overall very enjoyable. Aside from a general Winter Holiday theme, the stories don't have much in common--they really run the gamut--so everyone can find at least one story that they really enjoy.
FREE! It's free, it's long and it's various authors, many who are published (aka, competent writers). Overall it is a very enjoyable set of stories, most of which have happy Christmas story endings. This is what they're meant to be and they fulfill that role perfectly.
Being a free work compiled by a large number of authors, the writing quality varies between the stories. For the most part this isn't a problem, but for a few it was more noticeable and is the greatest weakness of those stories. Along these same lines, the topics, which vary greatly, won't appeal to every reader, but it's easy to skip any story that isn't to taste.