Random number generator
The RNG is sometimes referred to as the Random Number God. The RNG plays a pivotal role in determining the fate of all characters, and is sometimes considered the deadliest enemy in the game, despite the fact that expert players such as marvin have demonstrated that nearly every game is ascendable.
Although NetHack relies on the C rand() function, random number generation is not as straightforward as picking a number between 0 and n. Rnd.c holds all of the game's RNG functions, which can return results that depend on your Luck or experience level. Source divers might profit from examining these functions.
It was found recently by a player named Adeon that a sufficiently determined player can find the seed of RNG after starting the game and then use it to his or her advantage - for example to gain unlimited number of wishes from a fountain. For demonstration see his ttyrecs on NAO from 2009-02-28 (starting with http://alt.org/nethack/trd/?file=http://alt.org/nethack/userdata/A/Adeon/ttyrec/2009-02-28.10:31:15.ttyrec.bz2#). See also Sartak's blog entry Predicting and controlling NetHack's randomness and ais523's description of a game relying on extreme RNG manipulation.