Newsprint pad for life drawing, 12x18 and 18x24.
Moleskine drawing sketchbook.
Moleskine watercolor sketchbook.
Strathmore 400 Series Toned Mixed Media Pads for gouache painting from life, 6x8 tan, 11x14 tan.
Arches watercolor blocks (I prefer the cold press surface), 10"×14" 140 lb and 14"× 20".
Strathmore 500 Series Premium Charcoal Pads, 12x18 and 18x24.
Also, Cottonwoodarts.com has some new lines of plein air painting pads/sketchbooks that I've started experimenting with and seem really good so far.