Guardian Film has a list of actors who were considered for roles they didn’t get:
There is a famous example:
Tom Selleck (Indiana Jones): Selleck declined because of his commitments to Magnum PI. The part went to Harrison Ford.
A more recent one:
Ben Affleck (Brokeback Mountain): One of the actors whom director Ang Lee talked to about starring in the love story between two ranch-hands. The part went to Heath Ledger.
And two that get the head spinning:
Kevin Costner (The Matrix): Costner was apparently considered for the role of Neo (seriously). The part went to Keanu Reeves.
Chevy Chase (American Beauty):Turned down the lead role of Lester. Jeff Daniels was also considered for it. The part went to Kevin Spacey.
They also link to website that has many, many more examples. (Link via Movie City News)
> Guardian article
> The Top 25 Rejected Movie Roles at NotStarring.com