An interesting take is to scan aan empty film frame and use this noise frame as an overlay. In the following photo, I have moved the grain layer halfway to the right so that you can see the effect more clearly:
http://data.zmotula.cz/img/grain.png
This is how the grain looks in detail (you probably want to open the image to skip the compression and resizing artifacts):
http://data.zmotula.cz/img/grain-detail.png
This looks very nice and natural, as you get all sorts of small blemishes and “grain islands” that appear on film. On the other hand, the grain is always the same on all your pictures, unless you shuffle the overlay with each use somehow (rotate, shift, etc).