Optical viewfinder for the LCD screen. Pretty much a must if you want to do any handheld video work (and with the 5D, who the hell doesn't!). Gives you good critical focus and helps keep that baby steady.
Joby Gorillapod SLR-Zoom Flexible Mini-Tripod
Haven't actually used this heaps yet. I'm sure it's just waiting for it's time in the sun... or ebay.
Wii wrist strap.
much, much better than a big, neck strap and, if you have a wii at home, free!
Media 8-in-1 Usb Flash Card Reader
Essential. Importing from the camera drains the battery. This little guy does it in a "flash" (raw files still take a while). Need, need, need,