I work in 3 different mediums. Digital, clay and "junk." It's hard to pick my favorite tool, but for non-traditional work, my favorite tool would have to be Adobe Illustrator (and yes, it is a tool - that's directed towards those non-art people who think the computer just does it all for you - grr!). I like how I can move things around easily and enlarge as needed since it's vector.
My favorite traditional tool has to be color pencils. I use them in my clay and junk work. I use my huge set of prismacolor pencils from college. I remember saving up for them and was so thrilled when I finally brought them home. What are your's?