I've grown increasingly curious as to what supplies other artists use for illustration. To reciprocate, here's what I use.

Onsite:

Right now, I'm still working through a Molenskine Sketchbook (5" x 8"), but I also really like the pocket size Moleskine Watercolor book (3.5" x 5.5"). I can't wait to get back into this size after I finish the sketchbook. Though I agree with others that the quality is not really worth the price, I like the assurance that I will always be able to find these exact books anywhere I go.
Bottle of water.
Ziplock bag containing Faber-Castell pens (sizes XS and F), 0.7mm mechanical pencil, office and artist erasers, and 4 to 5 small watercolor round brushes (as long as they don't shed bristles, I'm not picky on the brand).
Winsor & Newton 14 half pan travel set...which I never clean.
Paper towel for dabbing...or sometimes just my pants.
My camera phone to snap a picture if I plan to finish the watercolor at home.
Oh, yes, and my dog.

At Home:

For personal stuff, I use the Moleskines. For clients, I like using Arches Cold Press at least 140lb watercolor paper. I buy 22" x 30" sheets which I cut down to size. This stuff IS worth the price.
Water glass.
All the same pens, pencils, and brushes.
Winsor & Newtown 45 half pan set...which I also never clean.
Swatch reference for my watercolors.
Paper towel.

And that's it. I'm not a tester of supplies, so I pretty much stick with something until it's no longer good for the job. These are all still doing the job!