Kriegspiel and "Wild 5"

I think the best variants are Kriegspiel and what both FICS and ICC call "Wild 5".

Wild 5 Rules:  White's pawns start on the 7th, pieces on the 8th.  Black's pawns start on the 2nd, Pieces on the first.  Each pawn is 1 move from promotion.  There are only 4 legal moves at move 1, the 4 knight moves.

That said, there is a known early strategy.  If White moves his Queen's Knight first, either Nf6 or Nh6 (don't forget, K starts on the d-file in this game, as the Queen starts on her own color, e8 is Light), then Black MUST play 1...Nc3 to answer 2.Ng4 (or 2.Nf5 after 1.Nh6) with 2...Nd5 to stop 3.Ne3# (don't forget, pawns are moving FORWARD, e3 isn't covered!).  So a typical start is 1.Nf6 Nc3 2.Ng4 Nd5 3.g8=Q OR 1.Nf6 Nc3 2.Ng4 Nd5 3.g8=N.

Kriegspiel Rules:  White can only see White's pieces, Black can only see Black's pieces.  White makes a move.  If it's legal, great.  If not, the server won't allow the move.  It could be illegal because a pawn tries to move forward on an occupied square, a non-knight tries to jump, or the move would put you in check.  Once a move is made, if a message is necessary, that message is given to both WHITE AND BLACK!  Such messages include:

x Pawn Tries (where x is a number) - This indicates the number of legal pawn captures available to the player on move.  So if it's White to move, and he has pawns on d4 and f4, Black has a piece on c5 and pawns on e5 and g5, White has 4 pawn tries (dxc5, dxe5, fxe5, and fxg5).

Knight Check - The King is checked by a Knight
File Check - The King is checked on the file he's on.  A rook or queen can cause this
Rank Check - The King is checked on the rank.  A rook or queen can cause this.
Short Diagonal Check or Long Diagonal Check - This can be caused by a Pawn, Bishop, or Queen.  A pawn is a diagonal check, not a pawn check.

To clarify Short vs Long diagonal check, it's short vs long based on the King's location, NOT the checking piece.  Therefore, let's say the White King is on b1, and the Black Queen checks from e4 via the move Qe7-e4+.  Both players would get the message "Long Diagonal Check", because from the King's perspective, the b1-h7 diagonal is longer than the a2-b1 diagonal.  It doesn't matter that from the Queen's perspective, it's the shorter diagonal (b1-h7 is shorter than a8-h1), the message will still read "Long Diagonal Check"

Lastly, if a capture is made, all it says is "Pawn Captured on d4" or "Piece Captured on d4".  It won't tell you what you grabbed if it's a piece.  So moving Nd4 might result in the following messages:

"Piece Captured on d4"
"2 Pawn Tries"

Where Black had say, a Rook on d4 with Black pawns on c5 and e5.