TV Lines

Step 1
First, create a new document that is 1 pixel wide x 2 pixels high.

Step 2
Next, zoom in all the way and select the pencil tool with a 1 pixel tip. Make a black dot on the bottom half of the image like below.

Step 3
Now, go to Edit -> Define Pattern... and name it tv lines. Now we have what we need to create the effect

Step 4
Now we need to open the image we want to add the effect to. Here is the image I used in case you don't have one (just right-click on it & save it).

Step 5
Next, add a new layer named "tv lines". Go to Edit -> Fill. Select "Pattern" from the dropdown list and select the pattern we created earlier. Set the Blending Mode to Overlay and the Opacity to 75%. You can play with this to get different depths, darknesses, etc...

Here is what the final image should look like: