The monthly Games with Gold freebie list is out for May, and it has a nice selection of Xbox One and Xbox 360 titles available. Between the two platforms (with all playable on Xbox One), you can download Peggle (360), Grid2 (360), Costume Quest (One) and DG2 (One) between May 1st and Jun 15th. In addition, if you previously missed it, Sunset Overdrive for Xbox One is still available for free until May 15th.
Peggle: May 16th - 31st
Grid2: May 1st – 15th
Costume Quest 2: May 16th – Jun 15
DG2: May 1st - 31st
Sunset Overdrive: Apr 16th – May 15th