While most homes have an old scanner hanging around, there have been a few situations in which I've needed one and had to improvise. Any modern DSLR or point-and-shoot will do a good impression of a scanner when you don't have one on hand.
When you're getting ready to try this, it's best to use a DSLR if it is possible, with a dedicated flash and a lens with some measure of zoom (18-55mm or 18-200mm works well). The nice brightness of a standalone flash will be almost the same intensity, if not brighter, of that of a document scanner. In order to do this properly, you'll need to snap away in a brightly lit room, which will allow you to eliminate your shadow, if you're careful enough.
Place the document on a desk. Then, step onto a chair. Try to be almost directly on top of the document, without causing a shadow. With appropriate lighting, this shouldn't be a problem. Then, take a few shots. Instead of just taking one, I've found that taken at least four shots will ensure that at least one of them will be properly in focus.
Once you've taken a few photos, it's time to resize them to a decent size using Adobe Photoshop, Elements, or any other image editing software. When you're in that program, try to resize the image to something like 1,000×1,500 pixels. I've found that this makes the 'scanned document' manageable enough so that anyone can receive it in their email.
It's important to be properly aligned with the document, otherwise you'll end up with slightly skewed shots, which could be a problem. You could also use some kind of brackets or holding arms in order to fasten the camera in place while you take a photos. Most tripods won't tip down enough for this to work, but if you place the document on an easel, it would be feasible to find the right angle for alignment.
[images via Andrew Hazelden and Kyle A. Koch]