Civilization
Cheat mode (DOS version):
Hold [Shift] and type 1234567890 to enable cheat mode. This allows you to view enemy cities, unfortify their units, sell their improvements, rearrange their resources, and view the power chart and the replay. 

Random enemy leaders (DOS version):
Press [Alt] + R. 

Full map (DOS version):
Press [Shift] + 5 + 6 during game play, then select any location on the map. 

Unlimited turns (DOS version):
Move every unit except for one. Then, save and quit the game. Reload the game to start with your turn will all units that last moved still in position and able to move again.