Let’s see… there’s a bunch, but my fav’s are:
Mike Myers
Alex Trebec
William Shatner
Paul Schaffer
Pamela Anderson
Peter Jennings
Dave Foley and the other Kids in the Hall
The scary thing about Canadians is that they look just like regular people and can often pass themselves off that way indefinitely. It makes one really wonder the truth about those around you …
Mary Pickford . . . Beatrice Lillie . . . Norma Shearer . . . Louis B. Mayer . . .
“Golden Age” Canadians:
Marie Dressler
Walter Houston
Yvonne de Carlo
Glenn Ford
Raymond Massey
Walter Pigeon
Deanna Durbin
Mack Sennet
Jay Silverheels (“Tonto”)
Hume Cronyn
Ruby Keeler
Christopher Plummer
of course, “America’s Sweetheart,” Mary Pickford
Fay Wray
Mort Sahl
Raymond Burr
Directors:
James Cameron
Jack Warner (Producer, of Warner Brothers fame)
Norman Jewison
Atom Egoyan
David Cronenberg
and, of course, Phil Hartman 
MACK SENNETT
KEANU REEVES
DONALD SUTHERLAND
JAMES DOOHAN
JOHN CANDY
EUGENE LEVY
ANDREA MARTIN
MARTIN SHORT
CATHERINE OHARA
RYAN STILES
COLIN MOCHRIE
AND THE GRAND DADDY OF THEM ALL
LORNE GREENE
Face it We are taking over… think about it Lorne Greene Green Card. Connection?
(See “The Canadian Conspiracy”)
Jim Carrey
Graydon Carter (former editor of SPY and current editor of Vanity Fair)
Isn’t Our Lady Peace canadian? Or am I thinking of another band?
Actually, Ryan Stiles was born in Seattle, WA. He and Colin make a dynamite WLIIA team, though.
RR
And my personal “fave”
Celine Dion - EEEEK!
Until her debut, I thought the US and Canada were allies, you folks have a lot of explaining to do up there!
Ok he was born in the States:
He later Worked with Second City in Toronto so while an American by Birth I’ll classify him a Canadian by upbringing.
(the giveaway is during his stint on the British version of WLIIAW when he was able to describe Saskatchewan the way only a Canadian would)
Michael J. Fox is still a Canadian to me.
screech (who has a penchant for Canadian guys)
Someone already mentioned Martin Short, but no one has mentioned Alan Thicke, another great Canadian of whom Martin Short does the best impression ever. Really. You’ll totally lose it.
Bryan Adams
Godspeed You Black Emperor!
Hawksley Workman
Loudon Wainwright III
Alanis Morisette (mucho sucky, but we won’t blame Canada)
Dan Akroyd
I read an interview with Ryan Stiles recently in which he said he considered himself half-and-half American and Canadian.
I love my Whose Line boys.
Leslie Nielson
Neil Young
The Guess Who
Keifer Sutherland (might be born in the states)
Steppenwolf
The Band
Shanya Twain
Abe Vigoda - Canadian!
Not just Lorne Greene, but Tom Green too!
And kingpengvin, thanks for mentioning the cast of SCTV!
Joni Mitchell
Rich Little
Paul Anka
Bret "The “Hitman” Hart
Jason Priestley
And a few more from the contemporary pop-music world:
Nelly Furtado
The Barenaked Ladies
Nickelback
Our Lady Peace
Jann Arden