It's impossible to resist this thread. Some pretty standard stuff from me:
Gabriel Garcia Marquez - One Hundred Years of Solitude
Milan Kundera - The Unbearable Lightness of Being
John Irving - The World According to Garp
(This is the novel that got me reading, the one that woke up my 'adult' soul.)
James Joyce - Dubliners
And one for the Canadians, possibly my favourite writer:
Robertson Davies - any of the trilogies.
It's hard to say how these have influenced me, though John Irving certainly has.
Having seen Andrew Black's post, I must add the Grapes of Wrath. It's as relevant today is it was in the 30's.