cs-tween
Easing/interpolation/tween functions in C#. And coroutines for using them with Unity.
I prefer this style of single-argument easing because it's simpler to switch which function you're using and the functions are simpler to think about. They're in terms of progress: mapping the input linear progress to an output nonlinear progress.
Alternatives
If you prefer multi-argument easing functions (that take a time, initial value, delta, and duration) see these Easing functions.