Krita

Last update: 26/09/19

Multiplatform illustration software.

Brushes

Hold shift and drag on the canvas to adjust the brush size.
E sets the current brush to eraser mode.
M mirrors the canvas.
Shift + W enters Wrap Around Mode. This is really handy for making tiled images.
Go through Settings > Dockers and test out the various dockers. I suggest having tool options enabled.
Go to New and select L*a*b*/Alpha as the colour mode. This colour mode blends nicer when painting, but is more performance heavy.
Hotkeys can be changed under Settings > Configure Krita > Keyboard Shortcuts [and] Canvas Input Settings.
Clipping layers work differently. They need to be in a folder, and they clip to everything in the folder that's below it.
The magic wand tool is split into two tools. The regular one does everything but select an entire colour regardless of placement, while the eyedropper tool besides it is specifically for that.
There's no settings to adjust the sharpen filter. I suggest flattening the image, duplicating it, applying the filter on the duplicate, then adjusting the transparency.
To add masks such as filter masks and transparency masks: Right click on a layer > Add
Layer modes (the drop down above layers) has some really cool effects under Mix. Click the checkbox to the left to pin favourites to the top. There's one with a cool name under Negative.
For professional printing, the default canvas sizes do not take bleed into account. Grab a printing template off the Krita forums.