Note: This code will only work when your health is not full, and you have exactly one Large Medipack. Press ESC to open the inventory screen, highlight the Large Medipack, then hold LOOK ([Keypad 0] by default) and press 9 on the top row of the keyboard. Then, use the Large Medipack once. Exit and re-enter the inventory and you should have unlimited Large Medipacks.
Do a somersault jump (Jump while pressing Walk forward) from a high place. A great place to attempt this is in the very first Streets of Rome level, when you are above the awning. If you attempt to do a somersault jump from above onto the awning, she will dive head first and snap her neck when she lands on the awning, causing the game to end.
Note: This code will only work when your health is not full, and you have exactly one Small Medipack. Press ESC to open the inventory screen, highlight the Small Medipack, then hold LOOK ([Keypad 0] by default) and press 0 on the top row of the keyboard. Then, use the Small Medipack once. Exit and re-enter the inventory and you should have unlimited Small Medipacks.
To do a somersault, crawl to the very edge of a platform or ledge. While holding CRAWL, press Jump. Lara will somersault into the area below. This is more useful than having to turn around and climb down while getting out of a tunnel. This is also useful when getting into the water mill in the "Old Mill" level.
Instead of jumping forward or backward and pressing ROLL immediately, press the opposite direction. For instance, if you jump forward, immediately press Down and Lara will do a flip in the air. Do the opposite if you jump backwards.