This script simulates complete exposed 35mm film strips with frame numbers, lettering, overexposed sprocketholes and DX film edge barcodes.
Copy it into the scripts folder from GIMP, you will find it then under Filters -> Decor -> Photo with Sprocketholes.
DX1 and DX2 numbers can be encrypted here:
Here is another example:
Update1: Optional sprocket hole shade thanks to Frank Ludwig.
Update2: New menu entry from Debian. Now under Filters -> Decor.
For all scripts together, go here: