Snapping

Just as in Object Mode, you can activate the snapping tools on the 3D View header and also select the type of snapping you want: Vertex, Face, Increments, and so forth. If the Snap tool is active, when you move things around they’ll snap. In addition, while pressing Ctrl, you can move them freely. Alternatively, if Snap is disabled when you press Ctrl, things will snap while you move them.