Other layout programs provide a visual indicator for alignment with another object's left/center/right (or top/center/bottom). What this does is facilitate aligning objects either in position or size when using the mouse to move or size them. Currently, the same result is possible using PageStream's alignment functionality but for a more interactive approach familiar to users with other applications suggest the following:

Implement a third 'snap to' option, specifically 'snap to object', that would be exposed in the Layout menu, toolbar and preferences/view. This snap-to method would provide on-the-fly snap lines for objects on the current page using top, bottom, left, right, vertical center and horizontal center. These lines would be visible when within the snap-to distance to provide visual feedback that the alignment is available.

These systems usually incorporate a 'distance from' snap-to and visual indicator as well when an aligned object is a certain distance from the object to which it is showing alignment. This is to assist with consistent spacing between objects, a la PageStream's distribute function. Suggest implementing this as well as a conditional snap line (only used when an object being moved is otherwise aligned with another object). There are two ways such a snap-to distance could be derived: from a preferences setting and from implied distance of two other objects already aligned.

Both methods have their points and both could be implemented with a checkbox to activate the 'implied' distance and a non-zero value for the snap-to distance otherwise activating it.