The guide.

In this guide I'll try to explain anything that is to do with Powder Toy. I'd appreciate people writing comprehensive element descriptions and tutorials to add here. You will be credited. Well, here goes...

The download

Whoever wants to play The Powder Toy, must download it. To do that, click here.
Please ignore the For Windows part. I haven't got much time and will blur it later.
After the download, you'll get a file named powder--.zip (or tar.gz for Linux or Mac).
Unpack it with your favorite unzipper.
In the folder you unzipped it to should've appeared two files: powder.exe and powder-legacy.exe. Run powder.exe to start the game. Press CTRL + i to install the game(optional). After the first run, there will be an additional powder.def in the folder.

Hotkeys

Key Description
Caps Lock Specific Element Erase
Insert or ; (semicolon) Replace mode
TAB Circle/Square/Triangle Brush
Space Pause
Q Quit
Esc Quit
Z Zoom
S Save stamp (+ Ctrl when STK2 is out)
L Load last saved stamp
K Stamp library
C Cycle view mode
1-9 Set view mode
P Save screenshot to .ppm
F Pause and go to next frame
H Show/Hide HUD
D Debug mode (+ Shift when STK2 is out)
I Invert Pressure and Velocity map
T Vine mode
W Toggle gravity modes (+ Shift when STK2 is out)
Y Toggle air modes
U Toggle ambient heat
R Reset LIFE generation (when in Debug mode)
~ Lua console
= Reset pressure and velocity map
[ Decrease brush size
] Increase brush size
B Edit decoration layer
Key combinations
Key Description
Ctrl+C/V/X Copy/Paste/Cut
Ctrl+Z Undo
Ctrl+G Toggle display of Newtonian gravity field
Ctrl+B Toggle display of decoration layer
Ctrl+Cursor drag Rectangle
Shift+Cursor drag Line
Shift+Alt+Cursor drag Line snapping in steps of 45°
MMB / Alt+LMB click Sample element
Mouse scroll Change brush size
Ctrl + Mouse scroll Change vertical brush size
Shift + Mouse scroll Change horizontal brush size
Shift + Ctrl + R Horizontal mirror for stamp (need stamp preview up)
Ctrl + R Rotate stamp counterclockwise (need stamp preview up)
Ctrl + Arrow keys Shift stamp in the direction of arrow key one pixel (no walls)
Ctrl + L alternative button Select subject for Specific Element erase
Ctrl + = Reset Electricity
Ctrl + Shift + LMB/RMB Fill

Installing Powder Toy.

Installing The Powder Toy allows you to double-click stamp or save files to open them. Install The Powder Toy with CTRL + i.
In development.

edit: @Xetalim stop digging up old stuff
also: don't follow this