Scan Lines

Open up a new document.

Open the document that you want the scan lines on.

Find the Channels Palette and make a new layer.

Zoom in (Ctrl + "+") to 1600% and taking your Pencil size 1 draw a small line. You can hold Shift and drag the pencil sideways; this will give you a straight line.

Grabbing your Rectangular Marquee select the line and 1 px under it.

Next go Edit > Define Pattern and give your pattern a name. Now delete the Channel layer you were just on and make a new one and Zoom Out (Ctrl + "-").

Grab your Fill tool and make sure the Fill is on Pattern with the pattern you just made selected (check the top of your window.. there's a drop down menu). Fill your Channel layer.

Still in your Channels Palette select the Load Channel as selection and change over to your Layers Palette.

Make a new layer.

With the colour that you want for your scanlines in the foreground, press Atl + Backspace. This should fill the marquee selection with colour. In this case, it is black. Deselect (Ctrl + D).

This next part is to your preference. You can lower the opacity, or blur the lines using Filter > Blur > Gaussian Blur. You can also play around with the blending modes until you get something you like. Here I Gaussian blurred by 0.2 and set the blending mode to Overlay. Click on the image to see the full picture.