Scanlines

Create a new image, 1 pixel wide and 2 pixels high with a white background. Increase it's size to 1600%.

Grab the Rectangular Marquee Tool (M) and drag in a selection covering half of the image. Go to Image > Adjustments > Invert (Ctrl+I).

Go to Select > All (Ctrl+A) and then to Edit > Define Pattern. Name the pattern in the dialog box and hit OK.

Close the image without saving.

Open the image you want to apply the scanlines on. Grab the Paintbucket Tool (G) and head up in the options bar. Select Pattern from the drop-down list, then choose the pattern you created from the dialogue box next to it.

Go to the Layers palette and press the button to create a new layer. Use the Paintbucket Tool (G) to fill this layer.

Back in the Layers palette change the opacity to 70% and the blend mode of the layer to either Multiply or Soft Light. The blend modes generate different results; experiment and see what fits your need the best.