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.