This free add-on adds Additive Animation Layers to Blender, using the NLA Editor.
What layers allow you to do, is create simple "offset" animations on top of previous animations.
Without using layers, if you want to make changes to an animation, you would have to repeat changes for almost all the keyframes
Here's a video showing Additive animation layers with this add-on:
And here's an example of what would happen if you tried the same process without it: