How much of a part do you think fate and/or destiny plays in life?

In the movie Signs the death of Mel Gibson's wife, his son's asthma, his daughter's miriad glasses of water and his brother's failed baseball career all turn out to be positive things which help him and his family overcome an alien attack.

Oedipus is abandoned as a baby in the hope of cheating destiny, and later tries to extract himself from his fate but cannot prevent the realisation of his ordained path, killing his father and marrying his mother.

Macbeth tries to encourage his fate as predicted by the witches (to become king) by murdering his way to the top, but Shakespeare shows that destiny will always find a way to bring us down should we mess with it, and Macbeth is indeed killed as forests start moving about and men not-born-of-woman appear. Clever stuff, fate.

But is it all just literary musing, or does fate and/or destiny have a real hand in life?