Pen: (Q) Left-click to affect single voxels.
Line: (W) Creates a straight line by left-click then dragging the mouse and release.
Screen Select: (E) Left-Click and drag to modify voxels in a rectangle from the perspective of the screen.
Block: (R) Affect block of voxels using left-click then drag to size and release.
Bucket: (Y) Left-click to affect contiguous voxels by 'filling' them with the same colour.
Picker: (I) Select the surface colour of a model.