Call of Duty: World at War and Call of Duty 3 were both made backwards compatible on the Xbox One this week, making a total of four from the Call of Duty backlog. Call of Duty 2 and Black Ops have been backwards compatible for a while.
Hopefully, this will renew World at War's online community, which has long since devolved into hackers and people flying around the maps. Regardless, World at War is, in my opinion, one of the best in the series, and is certainly worth revisiting, especially in light of what the series has become.