Personally...
Tripod: Nice to have and I'm using it about once a month.
Cleaning cloth: (Cheapest solution are the kind of cloth for cleaning glasses) Not so expensive and very nice to get finger prints and water drops from the front element.
Battery grip: A camera without one doesn't really feel right in my hands. I'm used to using it for portraits; it helps keeping the camera straight, offer twice the battery life without changing... but it is expensive and big, so with a Nova 190 / 200 bag I don't think it would fit.
Flash: I'm using mine very often, an indirect flash is pretty handy and can't be done with internal flashes.
50mm 1.8: I used it very often but it's getting less since I have the 100mm 2.8L. Mainly product photography is what I still do with this lens (and two flash guns)
UV filters: Only used for protection of the front lens element and the good ones that don't reduce the image quality are expensive. I have some for a few of my lenses.
Polarizers and ND filters: It's the easiest and in the end cheapest (although looking expensive in the beginning) way to get the biggest ones there are and hold them in front of your camera instead of attaching a fitting one on every lens. The only negative thing about it is that you need your left hand for that and so it's easier if the camera is sitting on a tripod.
Monopod: Don't have one. Not expensive, small, light, could be handy sometimes but I didn't *really* miss one. Does the job of an IS if used with non-stabilized lenses.