Retime: An Animation Scale That Doesn't Suck is a Blender add-on for retiming animation. Rather than scaling by percentage, it works in frames, so you set values the way you think about timing. When you retime a section, it moves the items that follow, keeping your animation tight and your markers useful.
You can edit while the animation plays, since the tool works independently of playhead position. It also carries options for customizing behavior, such as automatically adjusting the total animation length. Version 1.1 added Shapekey and Grease Pencil support along with quality of life improvements, extending the workflow to shape key and 2D animation.
The add-on runs across a wide span of Blender releases, from 2.92 through the 4.x series, and is released under the GPL license. Documentation is available, and the developer welcomes bug reports and suggestions directly. It's a focused utility for animators who want frame-based retiming instead of percentage scaling in Blender.