The X Mark

The X Mark is something the first six Straw Hats (Luffy through Chopper), as well as Nefertari Vivi and Carue, had on their left forearms.
During their adventure in Alabasta, they had to face a very tricky opponent, Mr. 2 Bon Kurei, with his Devil Fruit Power, which can make him look like anyone of them. Zoro came up with an idea to counteract this power.
They made a black X mark on their left arms, as a symbol of their belonging to the Straw Hats. To prevent this being copied by Mr. 2, it was covered by a bandage, and only revealed as proof of identity. The intention being that the band appeared to be proof, but in fact concealed the true sign. Luffy then stated that the X mark was a symbol of their friendship.
The X mark came into play three times in Alabasta:

1. When Carue reached Alubarna to deliver a message to Cobra, Chaka noticed his bandage and, thinking Carue was wounded, tried to examine it. Carue promptly slapped his hand away.
2. When Vivi is heading toward Alubarna, the capital city of Alabasta, she met Usopp but she wasn't sure if he was him or not. As she asked him to show their recognizing symbol, he showed the bandage, but without uncovering the X mark. Vivi understood that he was Bon Kurei disguised as Usopp and ran away.
3. When the Straw Hats were leaving Alabasta, they could not say good bye to Vivi in a proper way, to avoid the princess being recognized by the marines as their crewmate. So, they stand with their backs facing Vivi, raising their left hands and showing their X. Vivi and Carue replied in the same way too, as a symbol of their unbreakable friendship tie.