Unlock BFG weapon in 1st mission
If you pre-ordered the game, you received a card with a code printed on it that would let you use the BFG weapon in the 1st mission. Here are the codes for those who didn't pre-order.
Enter any of these as your name in the profile screen. Make sure to input the - symbol as shown. If done correctly, you'll get to enter your actual name after the code.