ALT + A
The Align tool aligns objects to the world origin, the cursor, the active object, or the floor. It exposes to options to control individual axis alignment, orientation or scaling.
The tool can also be used to align one object in between two others.
Any number object objects
For Active alignment, any number objects
For in between alignment, exactly 3 objects with one of them being the active and to-be aligned object.
This video demonstrates a very early version of the Align tool, with some of the features described above still missing.
The Align Relative tool is used to create duplicate or instanced objects and align them relatively to target objects, as the current selection is aligned to the active object.
Object Context Menu
Any number objects, one of them being active