Gesture Drawing Tools
(intended for pixels but the color advice is super, super helpful for all work!)
(though to be honest you might just want to go through her entire tutorial folder)
Art Tutorial (extremely in-depth, covers most if not all bases, probably my favorite or among them on this list so far)
Art Books by Andrew Loomis (pretty much a necessity for any artist)
Have a suggestion on what to put here? Let us know!