Wow this is something many many people didn't know. Comedian Chris Farley was the original voice of Shrek. 

Back in the mid-90's the makers of Shrek had one man in mind to voice Shrek. Chris Farley I guess had voiced 80% of the movie before he passed, making the only logical replacement one of his fellow SNL cast member's. They went with Mike Meyers and he did an outstanding job.