Guide Snapping

PhotoShop: To have guides snap to the ruler’s units while dragging, hold SHIFT while dragging a guide. To have it snap accutately to every pixel of an image, set the ruler units to pixels and zoom in about 500% so you can see every pixel in the ruler.

To have the guides snap to a layer, just select that layer and drag the guide over it. The guide will snap to the left/right sides, and the top/bottom sides. Plus, the guides will also snap to the horizontal and vertical middle of the layer. However if you have a selection, the guides will snap to the selection  in the same way by default, and ignore the layer boundraries.

To have the guides completely ignore all snapping (like snapping to guides, layers and selections) hold the ALT key while dragging a guide.