When you're in the game, press the ~ (tilde) key. It's typically in the top-left area of your keyboard, to the left of the 1 and below the Esc. A > prompt will appear near the top left of the screen and you can type in any of the following.
\givenewitem item - Add item to inventory -- see item list\givemoney # - Increase money by # amount\addexp # - Increase experience by # amount\addskillpts # - Increase skill points by # amount\addattribpts # - Increase attribute points by # amount\addbeing_afriend being -1 -1 - Add being to map at player position -- see being list\tdolby - Use science skill to affect enemy vision, if female character
Note: not all these beings are actually helpful to add, some may not work at all, and adding beings may ruin a game.