One Piece Known Carriers of the Initial "D."

Will of the D.
Monkey D. Luffy	Gol D. Roger †	Portgas D. Ace †	Marshall D. Teach	Jaguar D. Saul †
Monkey D. Garp	Monkey D. Dragon	Portgas D. Rouge †	Trafalgar D. Water Law

In the series, a number of characters have the middle initial "D.". This mysterious initial has been called the "Will of D." or "D.'s will" by several characters in One Piece.

The initial was first pointed out in Oda's SBSquestions and answers corner, when he was asked what the D. in Luffy's name stood for. Oda just replied to read it as a D. for now and that he would reveal the truth behind it in time. This was the first time Luffy's "D." was brought up, as well as Oda's first hint toward a bigger mystery.

Kureha was the first to mention the "will of D." when she revealed that the true name of the late Pirate King was Gol D. Roger. She told Dalton that Chopper had joined up with a very dangerous man. 

More on the "WILL OF D" : http://onepiece.wikia.com/wiki/Will_of_the_D