All Weapons, Infinite Ammo, Medipacks:
Face Lara exactly north, then enter the inventory. Highlight Small Medipack and press L, Up, X, Up, R, Y.

Infinite LaserSight Ammo:
Combine the Crossbow or Revolver with the LaserSight, Zoom in, hold L and press Start. Select the Pistols and press A. Exit the pause screen. All while still holding L.