Aside from cameras and lenses, here is the stuff that sometimes finds its way into my camera bag:
GPS data logger - so I know where i was when I took the pictures.
Hot shoe bubble level - so that I know my camera is level
Close up filter - Good 2 element filters, not the cheapo ebay kind, so that I can take macro shots with normal lenses
Polarizing filter
Remote shutter release with intervalometer so that I can trigger the camera without touching it
Flashes and a flash controller and colored gels for the flashes
Cleaning stuff - blower brush, lens cleaning fluid, lens paper, microfiber cloth, sensor cleaning brush, dust-aid pads, rocket blower
ND filters - for getting longer shutter speeds.
Graduated ND filters - for reducing dynamic range. I use a cokin style holder so that I can adjust the "horizon" on the filters.
Consumables - batteries, memory cards
Extension tubes - for getting even closer macros
5-in-1 "reflector" (white, silver, gold, translucent, and black)
Flashlight - for finding stuff in the dark. I use an LED headlamp if I plan on shooting much after dark. Also, a bright flashlight for "painting" things in the dark.
A mini-voice recorder for taking "notes."
A focusing rail and leveling base for the tripod - the focusing rail is useful for focusing on macro work and the leveling base and focusing rail work well together for panoramic shots.
I'm sure that I'm forgetting lots of other random items that used from time to time, but that's a reasonable list of the main things that jump to mind.