Most commonly used and essential hotkeys - bookmark this post!
Navigation
Mouse scroll wheel = zoom
Alt + left mouse = tumble / rotate camera
Alt + middle mouse = pan camera
Alt + right mouse = zoom
Square brackets [ ] = undo / redo camera position
Spacebar (tap it) = switch viewports
Tools
W = move tool
E = rotate tool
R = scale tool
Q = select tool / hide manipulator
+ = increase manipulator size
- = decrease manipulator size
Actions
Ctrl + Z = undo
Ctrl + S = save
Ctrl + Q = quit
Ctrl + A = attribute editor
Ctrl + G = group
P = parent
Shift + P = unparent
X = snap to grid
V = snap to points / vertices
D = change pivot location
Ctrl + D = duplicate
G = repeat last action
S = set key on selected
Shift + W = key only translation
Shift + E = key only rotation
Shift + R = key only scale
Display
1 = standard form, as is
2 = smooth preview with wireframe standard form
3 = smooth preview only
4 = wireframe
5 = shaded
6 = textured
7 = lighting on in viewport