Board: 8x8, 64 cells
Pieces: normal set (cannot be played with a crafted normal set, though)
Related: fusion chess
Other: commercially sold at http://www.baumspiel.de
RulesIn a move, a player can also step on his own pieces. Pawns can also be stepped on and can step on other pieces. The king can do neither. Stacks of pieces may have arbitrary height.
If a stack is captured, all its pieces are removed.
A stack can move like the topmost piece (unlike fusion chess). A stack can split into two (stacks or pieces) by moving any number of pieces from the top. The order of pieces within a stack may never be changed.
Pawns promote if they are the topmost piece and standing on the opponents base array (line 1 or 8). This may be the result of splitting a stack that was standing there. Pawns promote to captured pieces only.
Additions I used to clarify circumstances not mentioned in the rules:
If there are no captured pieces, pawns do not promote. If the opponent makes a capturing move and still finds a pawn of your color on his baseline, he must promote it to the captured piece as a part of his move. If this puts himself into check, capturing is not allowed. If he finds more than one pawn on his baseline or captures more than one piece (i.e. captures a stack), he may select.
After splitting a stack, a pawn may remain on your own base array. From there it can move one forward. Pawns being on the second array may move two forward, regardless if already moved or not.
NotesI am not the author and am not affiliated with them. But the game is great and I think it should be mentioned here.
The set commercially sold is also suitable for playing fusion chess and for building pieces like the marshal or archbishop to enhance a standard set.