The cable release for sure; circular polarizing filters; hoods for lenses that didn't come with them; microfiber cloths; a charger that can be used from a car cigarette lighter socket; and when I dive into flash after my intended retirement next year, the off-camera shoe cord.
I have a BG-E2N, (Genuine Canon Model). It flexes and twists when tripod mounted, causing the camera to droop downward after you lock the tripod. It is made of plastic, and even with a 17-55 f/2.8 lens, it droops. Finally, I just removed it for tripod use(No drooping with it off, even with a big telephoto lens). It is a hassle to remove and re-install, so a built-in grip like the 1D Mark II or III has is much better.
