This is a list of movies which play with alternative possible realities, in various ways:

• It’s a Wonderful Life [Frank Capra, USA 1947]
• Przypadek (Blind Chance) [Krzysztof Kieslowski, Poland 1987]
• Lola Rennt (run lola run) [Tom Tykwer, Germany 1998]
• Sliding Doors [Peter Howitt, UK 1998]
• The Family Man [Brett Ratner, USA 2000]
• Me Myself I [Pip Karmel, Australia 2000]

On the topic of possible worlds, this post may be of interest.