Oh wait, only for testing!
No, sorry, I used "all weapons" in the final level too :_(
Open your console with ~ or \ (the keyboard button near TAB and 1) and write:
\save_game NAME
\load_game NAME
Put -DEVMODE in the end of your path (in the shortcut) and enjoy new commands:
F1: 1°/3° person
F2: move to next checkpoint
F3: new spawn point
F4: no clip
F9: save current position (fast save)
F10: load current position (fast load)
F11: see more debug information
P: all weapon in your inventary
O: set 999 bullets in the selected weapon
backspace: god mode
equals: increase speed
minus: decrease speed
F5: set default speed