Silver

Silver is a chemical element, which is greatly effective in killing beings like ghouls, and most notably - vampires.

A Weapon to kill vampires
In Hellsing, A bullet made out of Silver is often supposed in to be the only weapon that is very effective against ghouls and vampires. Anderson's Bayonets were also made out of Silver, which, Alucard commented about the fact that "these things" can actually hurt both true vampires, and FREAK, or Artificial vampires.

Symbolic Role
Silver production has also inspired figurative language. Clear references to cupellation occur throughout the Old Testament of the Bible, such as in Jeremiah's rebuke to Judah: "The bellows are burned, the lead is consumed of the fire; the founder melteth in vain: for the wicked are not plucked away. Reprobate silver shall men call them, because the Lord hath rejected them." (Jeremiah 6:19–20) Jeremiah was also aware of sheet silver, exemplifying the malleability and ductility of the metal: "Silver spread into plates is brought from Tarshish, and gold from Uphaz, the work of the workman, and of the hands of the founder: blue and purple is their clothing: they are all the work of cunning men." (Jeremiah 10:9)

Multiple crosses were made with Silver, notably, a Manchester Cross was melted onto the Casull to make the bullets, something Alucard stated in episode one of the original Hellsing anime.

Silver Bullet

 * Hellsing Combat Pistol 'Jackal'
 * Hellsing ARMS Casull

"Blessed"

 * Bayonets - Alexander can also summon multiple bayonets that are automatically blessed with Silver.