The official solution for sprite atlases for Unity.
Ranked in these QuestionsQuestion Ranking
Pro Create Custom Output Via Code
Set custom packing specified by code useful for those who have to follow a guideline on packed sprites.
Pro Comes built into Unity
The Pro license brings full 2D support within Unity without having to use an external program or plugin for creating atlases.
Con Doesn't work anymore
As of Unity 2017 it's completely broken.