Some parallel universe films:

(already mentioned)
It's a Wonderful Life
Sliding Doors

(others of recent vintage)
Me, Myself, I (with Rachel Griffiths)
Passion of Mind (with Demi Moore)
Family Man (to be released)(with Nicolas Cage)

variations:
The Double Life of Veronique (Kieslowski)

Shari L. Rosenblum

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