Excerpt from Wikipedia (http://en.wikipedia.org/wiki/Standard_candle#Supernovae)

Type Ia SN (Super Novae) are some of the best ways to determine extragalactic distances. [...] Because all Type Ia SN explode at about the same mass, their absolute magnitudes are all the same. This makes them very useful as standard candles. [...] Therefore, when observing a type Ia SN, if it is possible to determine what its peak magnitude was, then its distance can be calculated. [...] Using Type Ia SN is one of the most accurate methods, particularly since SN explosions can be visible at great distances (their luminosities rival that of the galaxy in which they are situated), much farther than Cepheid Variables (500 times farther). Much time has been devoted to the refining of this method. The current uncertainty approaches a mere 5%, corresponding to an uncertainty of just 0.1 magnitudes.