Scene Timeline is the best alternative to Unity's built-in Timeline Editor.
Timeline Editor introduced in Unity 2017.1 that can be used instead of this tool.
Unfortunately, it can be used only when your project requirement allows you to use Unity 2017.1 or later, and need to update existing scene structure ready for use of Timeline Editor.
Scene Timeline can be used without any changes to existing scene structure, and more friendly interface like Maya, MotionBuilder and other DCC Tools.
And also this tool works on older versions of unity, I've tested on Unity 5.4.5.
If you are not able to use Unity's built-in Timeline Editor due to production requirement and/or scene structure.
Scene Timeline is the best alternative for you.
