To get the following codes to work, Lara must be facing absolute North. One way to do this is to find a block or ledge that's facing north and climb it. You can confirm this by checking your compass at the Inventory screen. If the red portion is transparent, then you're facing North.

Level Skip:
Face North. Go to the Inventory screen and highlight 'Load Game'. Now hold H + E + L + P for a few seconds. Release the keys and close the inventory to skip to the next level.

Unlimited Items:
Face North. Go to the Inventory screen and highlight 'Small Medipack'. Now hold G + U + N + S for a few seconds. Release the keys to get infinite items.

All Weapons:
First enable Unlimited Items. Now face North. Go to the Inventory screen and highlight 'Large Medipack'. Now hold W + E + A + P + O + N + S for a few seconds. Release the keys and close the inventory screen, then reopen it to get all weapons.

Thanks to Revolution readers Patrick Kofler, Even Serd, Bruce Fuehr, Thorsten Höfer, Tom T. Chen, Ryan, stargazer, Ad Voerman, Marcel Maagdenberg, Eric, and Miguel!