Scan Lines

I recently saw on a website the tediously overused scan lines, except these went in a gradient down the page. How do I do that?

-={GeekGrrl}=-,
Mr Photoshop gets a lot of these how'd they do that questions. I enjoy figuring out how to create a certain effect, but I have to see it. Try to included a URL so I can better answer questions of this type.
To get to your question, I'd first create the scan line look. I made plenty of these into Actions that you can download for image effects if you have version 6.0. Then watch the History palette and you'll see how it was done. Then I'd try applying a gradient you like to a new layer and setting the layer blending mode to overlay.

But maybe your question was asking how to get an image to go through a large vertical transition as an HTML background image. As long as you want the image to remain consistent horizontally, you can create a really tall but very thin image that will tile horizontally. Though it will seem like a very large image at first, it's file size is actually small.