Taking good photos is about more than just the camera.
What accessories are you sure to take with you?
Good, comfortable footwear - many types of photography involve walking a fair bit, and you don't want your feet getting sore and being a distraction.
Similarly, suitable clothing in general - if there might be a great shot in the next field over, don't let fancy trousers stop you climbing a fence to have a look!
Oh and a friend - to warn in case that one last half-step to get the scene framed perfectly will result in a big splash. ;)
Thermos of tea. Because sometimes photography is about patience (and cold places) - it's hard to take good pictures when your hands are shaking.
Some coffee is good too!
Nikon 7072 Lens Pen Cleaning System — it's about $10.
It's the size of a fat pen, has a retractable brush on one side. The other side has a soft chamois-like material to remove fingerprints. Simple, portable, useful.
Depends of what you're shooting. For landscapes many people won't go out without a tripod. On the other hand tripod is quite useless for indoor reportage, in which you can perfectly use an external flash.
Batteries and memory card are handy for most applications, though.
Some sort of calibration thingy to include in at least one shot per "scene". There are a number of different makes & models from different makers -- I like the Lastolite collapsible XpoBalance and the XRite ColorChecker Passport -- but anything that will help you set a white balance at shooting time and set white, mid and black points in post will help immensely.
Oh, speaking of Lastolite (as one brand, again, among many good ones), it profiteth not to venture forth shooting people without a reflector. I usually carry a collapsible white/soft gold model with me, even for "just walkin' around".
Polarizers are very useful when shooting outdoors, especially because they can double as neutral-density filters.
All good suggestions ... I would like to repeat the 'extra memory cards' thought.
I had a situation where I brought a single high capacity memory card, but the card had some kind of error on it and I was stuck. Luckily my wife had a spare, lower capcity, card with her ... so I could take some pictures (but I couldn't take them in raw+jpg the way I wanted).
gallon sized zip-lock bags
tripod (but only if I have a specific reason, usually this is left behind)
I always have a polarizer with me but it is a tool not generally useful. I take photos at bodies of water (lakes, ocean) and of varnished wooden objects (read: boats). A polarizer makes the difference between large burnt-out white areas and nice visible structure for me.