Option-click the wifi icon: special diagnostics info
Option-click the bluetooth icon: diagnostics info
Shift-Option-click the bluetooth icon: debug menu
Hold option while resizing a window: resize from both sides at once (see gif: https://twitter.com/dceddia/status/859518600931143680)
Option-click the sidebar icon: turn notifications on and off.
The option-resize window trick is crucial when you come across a monitor/video card bug that doesn't let you touch the edge of the window that you need to drag in order to bring the window on-screen. Just option-drag the opposite edge as a workaround.
- option + click on audio -> choose audio in + out
- option + drag on columns in finder -> resize all columns
One I find useful is option-right-click dock icon that force reveals the force quit menu item.