PDA

The PDA is one of Doom 3's most useful tools, besides the player's weapons.

It acts as a media player, email reader and access key to restricted areas. Besides the player's personal PDA, other PDA's can be picked up and scanned to read helpful hints (ie: backstory, hints to monster types ahead, and combination codes for lockers).

One fun PDA to look out for is the id PDA which contains special messages from the developers of the game. Look around vigilantly in the last level and find the seceret switch!