Clipboard and stamps
To copy, press Ctrl+C. To cut, press Ctrl+X. Then move the mouse to the top left corner of the area to copy/cut. Press and hold the left mouse button while dragging the mouse to the bottom right corner of the area to copy/cut.
To paste, use Ctrl+V. Move the mouse to position the clipboard contents (it will snap to multiples of 4 pixels), then click to place it. Before placing it, use Ctrl+R to rotate it, Ctrl+shift+R to flip it horizontally, or Ctrl+arrows to move it by 1 pixel at a time.
Stamps are like an extended clipboard, and are saved to disk. The clipboard is cleared when Powder Toy is closed, but stamps are not. Up to 240 stamps can be saved - if you make more, then the oldest stamps are deleted to make room.
To create a stamp: Press 'S'. Select the area in the same way as when using the clipboard.
To use a stamp: Press 'L' to load the most recent stamp. Place it in the same way as when using the clipboard.
Stamp browser: To use a stamp other than the most recent one, press 'K' to show the stamp browser. Click a thumbnail to use that stamp, or click the red minus sign in the top right corner of a thumbnail to delete it. Change pages in the stamp browser by clicking the arrows at the bottom of the window, or by using the scroll wheel.