Intended for use with orthographic setups but actually also works in perspective with just a little tweaking.
Add a single monobehaviour to any gameobject to turn it into an element that positions itself relative to a main camera according to some simple triangle math, achieving a parallax effect.
Group gameobjects together and apply the monobehaviour to their parent to create an entire parallax layer--use cases include parenting particle systems/animations to a static background image. (Think a house in the distance giving off smoke.)
Package comes with literally just one C# file--true out-of-the-box functionality!