Sodium Carbonate, also known as washing soda, soda ask or soda crystals.
 
Washing Soda is simply powdered sodium carbonate, and like sodium bicarbonate it is an alkali, however it is much stronger with a pH of 11. (As a comparison, bicarb has a pH of 9.)
 
Washing soda has a number of properties that make it especially useful around the home. It is particularly effective at:
 
Breaking down acids and oils
Softening water
Cutting through soap scum
Removing stains