I'm much faster with my good 'ol ballpoint pen than a trackpad and keyboard, so from time to time I do some hand-drawn concept art to help me quickly plan out my textures. Some of the in-game textures (almost all of the items) were traced from my own on-paper art! Hats off to whoever can read my sloppy note handwriting.
I had the idea of slimeballs being slime-cubes... but they are called slime BALLS, after all. Enchanted books were going to have custom CIT (MCPatcher) textures that gave them witty titles based on their enchantment, but I later realized that the writing would become illegible at lower res.
Concepts for the first three teirs of tools.
I've done a heck of a lot of doodles and designs for spiders (there's two or three other pages that pretty much look like this), mostly because it's a lot of fun drawing their goofy faces. I got the vanilla spider's face designs squared-away quite early on; most of these are ideas for spiders from various mods.