TexturePacker is an engine agnostic atlas creator with support for both Unity and the 2D Toolkit plugin. Quickly create atlases that will optimize packing by rotating sprites and allowing a breadth of other options in a simple interface.
Ranked in these QuestionsQuestion Ranking
Pro Simple Interface
Drag and drop sprites onto the canvas, change the settings, then output.
Pro Multiple Atlas Output Formats for Unity
There are three different outputs for Unity to read the atlas; JSON, Texture2D, and 2D Toolkit.
Con Pure functionality in the free version, you can't even set margins
The full price is too high for such a simple app.
Con No direct integration
TexturePacker is an external third party application with no Unity Inspector interface.