In many graphic and writing tools there snapping grids and other constraints which block the user to move graphic objects to unwanted positions or to help aligning objects to other objects.

In Linux (as I remember correctly) it's common to use the ALT key on the keyboard to allow the user to disable those features and enter the "free object movement" mode spontaneously while pressed.

What is the best key for such feature in a standard 2D graphics editor for Mac, Windows and Linux?