Krita
Multiplatform illustration software.
Brushes
Memileo Impasto Brushes (performance heavy)Sketch V2.
Dry & Dirty Bundle
RapidFire ETERNAL, a kit of Brushes for low-end Computers
Expressive Inks
Concept & Illustration V2
Cheap Ballpoint pen Brush
Aurelia's Lineart (comic?) bundle
Mikhailasmo's Inking Brush Set
mtPaint pixel art brush set for Krita (pixel brushes)
Tools
Plugin: Blender Layer - Live 3d View in KritaBrush SFX
Zen picker
Gradient tool & maps: resources (a link to various photoshop gradients and a tool to convert them)