Crazyhouse

Crazyhouse (also known as drop chess, mad chess, reinforcement chess, turnabout chess and schizo-chess) is a chess variant similar to bughouse chess, but with only two players. It effectively incorporates a rule from the game shogi, in which a player can introduce a captured piece back to the chessboard as their own.
Rules.
All the rules and conventions of standard chess apply, with the addition of drops, as explained below.
Unlike shogi, having two or more pawns on a file, and checkmating with a dropped pawn, are both permissible.
The physical problem of changing the color of a captured piece can be handled or averted by:
Notation.
An extension to the standard chess notation is used to record drops. Drops are notated by the piece type, followed by an @ symbol, then the destination square. For example, N@d5 means "knight is dropped on d5 from reserve".
FEN.
There is no standard FEN specification for Crazyhouse. However at Lichess an extended version of FEN is in use. Here is Lichess's FEN implemention example.
Lichess simply adds a 0th rank as a reserve. There are more than 8 pieces on the reserve, so the last section may have more than 8 characters.
A different notation is used by Xboard/Winboard. The reserve is given in square brackets following the board position.
Chess.com uses another notation. The reserve is put after full-move number.
To keep track of which pieces currently on the board are actually promoted pawns, Lichess and Xboard/Winboard use "~" after letter designation.
However, Chess.com uses coordinates of promoted pawn to resolve it.
Variations.
Minor variations of the rules have resulted in some variants.