Press [~] in the game to open the console and type in

 give all                - Get all items
 give life               - Get Life
 give health             - Get 100 life points
 pause                   - Pauses game
 toggle g_p_god          - Invincibility
 give jetpack            - Get jetpack
 give ammo               - Get ammo
 Give forcefield         - Get forcefield
 kill                    - Suicide
 toggle r_timings        ???
 toggle r_stats          - Game Stats
 toggle g_debug          - Debug Mode
 camera camera           - Ability to Search Whole Map
 camera player           - Normal Camera
 toggle g_map_info       - Map Info
 give keys               - Get All Keys
 give nuke               - 10 Nukes
 give secret             - Mark Secret Found