Lmb | Material 1 (red)
Rmb | Material 2 (blue)
Mmb | Material 3 (green)
Ctrl + Alt + Click | Material 5 (cyan) (for specific delete mode)
Alt + Click or Middle Click | Select / Sample On Screen Element
e | Element Search
Ctrl + Shift + Lmb | Add To Favorites

Tab | Cycle Brush Shape
Scroll | Brush Size
Ctrl + Scroll | Brush Size Vertical
Shift + Scroll | Brush Size Horizontal
] | Increase Brush Size
[ | Decrease Brush Size

Shift + Drag | Line
Shift + Alt + Drag | Line Snap
Ctrl + Drag | Area
Ctrl + Shift + click | Fill

Ctrl + c | Copy
Ctrl + v | Paste
Ctrl + x | Cut
r | Rotate Paste
Shift + r | Flip Paste Horizontal
Ctrl + Shift + r | Flip Paste Vertical
Arrow Keys | Move Paste

0-9 | Display Modes
b | Deco Layer Edit (also turns deco on)
Ctrl + f | Highlight Element (uses material 1 (red))
g | Grid modes
h | Toggle Hud
z | Zoom
z + Scroll | Magnify or Reduce Zoom
z + Click | Fix Zoom Window
Shift + 1 | Life Gradient Display (only in debug mode)

w | Gravity Modes (+ Ctrl when STK2 is present)
y | Air Modes
Ctrl + e | Edge Modes
; | Replace Mode
Ctrl + ; or Delete | Specific Delete Mode
d | Debug Mode (+ Ctrl when STK2 is present)
= | Reset Pressure
Ctrl + = | Reset Electricity
r | Reset Life Generation
i | Invert Pressure And Velocity

Ctrl + g | Gravity Field
Ctrl + b | Deco Layer
n | Newtonian Gravity
u | Ambient Heat
` or ~ | Show Console

s | Save Stamp (+ Ctrl when STK2 is present)
l | Load Last Stamp
k | Browse Saved Stamps
r | Rotate Stamp
Shift + r | Flip Stamp Horizontal
Ctrl + Shift + r | Flip Stamp Vertical
Arrow Keys | Move Stamp

Spacebar | Pause
f | Frame Step
F5 | Reload Simulation
Ctrl + z | Undo
Ctrl + Shift + z or Ctrl + y | Redo
Esc or Q | Quit
Ctrl + i | install powder toy

This was largely stripped from the wiki so all credit goes there
Wiki Hotkeys Page: https://powdertoy.co.uk/Wiki/W/Hotkeys.html