Halo: Spartan Assault is a download-only title that launched on Xbox One last year. Spartan Assault was originally release as a mobile game for Windows Phone and Windows 8 last summer before being ported to Xbox One in December (and, more recently, Xbox 360 and Steam). While enjoyable, the twin-stick shooter is on the short side and has limited co-op play, as noted in GameSpot's review.
Max: The Curse of Brotherhood is another download-only game released on Xbox One in December. It's a platformer with some intriguing puzzles, although GameSpot's review described it as "competent but largely unremarkable."
As announced last month, Microsoft has finally made good on its promise to bring Games With Gold--which gives away two free games per month to Xbox Live Gold subscribers--to its newest console. There will, however be some changes from the way things worked in the past; namely, the titles will begin being offered with "subscription-based access," which makes it sound as if the service will work more like the PlayStation Plus Instant Game Collection.