What are some truly convenient accessories for a camera that can really improve your shooting capabilities and quality? I was thinking a matte box, but are there any specific tripod suggestions, or shoulder rigs, dollys, etc.?
Once you have your camera and sound taken care of, I would start with an inexpensive fluid head tripod, dolly and slider/table dolly. Steady images will raise the "watchability" of your motion pictures significantly.
I own this tripod and dolly and they are all-metal and very good value-for-money.
Lighting would be next - and a matte box and shoulder rig would be last.