This script lets you overlay markers on the timeline. Its useful if you need to set keys on different objects at certain times.
I created a QWidget with a custom paint event which renders a vertical line at a list of positions. Then I installed a couple event filters to create a shift+click context menu over the timeline. I couldn’t use the widget’s contextMenuEvent or mousePressEvent or else it would interfere with Maya’s context menus.

download the script here