Who kills the Master in Doctor Who?

I can't speak for the classic series, but in New Who, the Master is shot by Lucy Saxon (his wife) and refuses to regenerate in Last of the Time Lords. However, at the very end of the episode, a woman is seen picking up his signet ring from the ashes of his funeral pyre.

In End of Time, the Master is resurrected. He attacks the Time Lord President at the end of the episode and disappears with the rest of the Time Lords.