2D Toolkit allows for different sizes of atlas from 1x, 2x, or 4x which can be changed depending on platform resolution.
Choose from either a box, sphere, or mesh collider that will output as either a 3D or 2D collider in Unity.
The documentation for the toolkit is well written and there are video tutorials as well as an active community.
Pixel games in the style of 8 or 16 bit systems need to have each pixel uniform on the screen which 2D Toolkit supports with a camera component.
2D Toolkit provides a completely proprietary sprite implementation. Though it can be used alongside Unity's 2D systems, the two are completely separate and require different code to utilize.
Set custom packing specified by code useful for those who have to follow a guideline on packed sprites.
The Pro license brings full 2D support within Unity without having to use an external program or plugin for creating atlases.