Yep, that is what I came in to say.
Also, Lena Headey did a great American accent on Sarah Connor Chronicles.
The French Canadian accent is incredibly hard to nail down. Anglo-Canadians will often try to mimic it, but unless they’ve spent most of their lives in Quebec, they’ll usually fail horribly. This is why I was quite surprised when Edward Norton completely nailed it in the otherwise forgettable The Score.
He just starred in a film directed by a friend of mine, where he had to put on an Australian accent throughout. I’ve only seen clips, but he’s very convincing.
2005 interview with Christian Bale from NPR’s Fresh Air. He talks about his accent near the end at around 16:30. (This is the mobile version.)
I remember reading a newspaper article a few years ago in which the paper asked Spanish linguists to critique the accents done by a variety of actors playing Latinos. Major surprise: they said Al Pacino did an excellent job doing a low-rent Marielita accent in “Scarface”. If memory serves, the reviews of the movie when it was released tended to savage Pacino for his “ridiculous” accent (among other things).
FWIW, the linguists didn’t care for his accent in “Carlito’s Way”. I think they said his accent was not bad, per se, but rather not ethnically correct (they said his accent did not reflect what a New York Puerto Rican of his background would sound like).
Similarly, Cary Elwes grew up partly in the UK and partly in America. Like John Barrowman, Gillian Anderson, and perhaps Bale, he is likely “fluent” in both accents.
Emma Thompson as a French princess in Henry V. And as American Nanny G on Cheers.
He also did a perfect English accent in “Reilly: Ace of Spies.” I think he also did a German accent in that series as well, but I don’t know how good it was.
The times I’ve heard him do a Russian accent (“Amerika,” The Hunt for Red October, I’ve been less impressed.
If we’re doing cable dramatic series, both Idris Elba and Dominic West (Englishmen) did great American accents (though probably not specifically Baltimorean) in The Wire.
Many are first made aware of their origins when they hear the actors’ audio commentary on the DVDs. Bryan Cranston spoofed this hilariously in the commentary for the first episode of the first season of Breaking Bad, when he starts out by saying, in an exaggerated Brit accent, “Hello! This is Bryan Cranston, and nobody knows I’m British!”. 
So why do you suppose he did the world’s worst New England accent in THE CIDER HOUSE RULES? He did it in his normal Cockney accent, playing a lifelong state of Mainer. I had to turn it off.
Kelly MacDonald whose Texas accent as Carla Jean Moss in No Country for Old Men sounded absolutely genuine. All the more remarkable since she’s Scottish.
Viggo Mortensen in Alatriste. His natural Spanish accent is Argentinian (he grew up there), and I’ve heard interviews when he was working in the LotR trilogy where his Spanish was rusty and he occasionally had to ask for a word which wouldn’t come to mind.
But if I didn’t know better, watching him play that fortune soldier from León I would have been completely sure that his lastname is actually something as Old Castillian as Castille herself, and that he was born somewhere between Zamora and Burgos.
Nice. How good is Mexican actor Gael Garcia Bernal at faking a Spain accent? (I think it was in an Almodovar film.)
If I didn’t know he was English ahead of time I might have missed it. But I did and his accent sounded off to me in the beginning. As the show went on it became flawless.
That is Poppy Mongomery’s accent. Listen to her do interviews. On Unforgettable she lets the Austrailian come through in some words. I don’t think she is trying to hide it. In other roles I never noticed it. I looked it up, she has been in the U.S. since she was 18. I think she is one of those people who easily take on the accent of where they are living.
I agree about Simon Baker. But even better in The Mentalist is Owain Yeoman who plays Rigsby. Baker has a strange affectation that doesn’t sound right but I think that is an acting choice rather than an accent problem. I had no idea that Rigsby was Welsh.
I did not think that Dominic West’s accent was particularly good. I did not know where he was from but I knew he wasn’t American when I first saw him. Idris Elba’s accent was perfect. But strangely I did not think his American accent in Prometheus was very good. But I would still watch him reciting the phone book. He has become one of my favorite actors.
Matt Passmore (The Glades) does a really good American accent. Joe Anderson (The River) did not.
I am a little fanatical about Band of Brothers. More than half the cast was British. Almost all of them did a great job.
Damien Lewis (Maj Winters) perfect
Shane Taylor (Doc Roe) perfect
Peter Youngblood Hills (Shifty) (no idea what his real accent is moved between countries) perfect
Nicholas Aaron (Popeye Wynn) perfect
Dexter Fletcher (SSG Martin) perfect
Robin Laing (Babe Heffron) perfect
Matthew Leitch (Talbert) perfect
Ross MaCall (Leibgott) perfect
Marc Warren (Blithe who the built an entire episode around) bloody awful
Simon Pegg (1SG Evans) well thankfully he only had a line or two
Usually when I hear a bad American accent it just sounds off to me. I usually can’t place where the actor is from it just sounds wrong. I think I am particularly tuned into it for some reason. I know I am one of the few that thought Hugh Laurie wasn’t perfect.
Oh honorable mention to John Barrowman. He does two accents perfectly, American and Scottish. But they are both his. Born in Scotland, spent most of his youth in America. Goes back and forth. His normal speaking voice is American but says when he is around family it switches on. I had a friend like that. He is from Guyana. You would never know he wasn’t born in America. Then I heard him speaking to his mother.
You and my wife could start a club. She swoons for that guy. Heck, I’d do him! 
I haven’t seen La mala educación so I can’t answer personally, but I don’t recall any criticism and you bet the critics would have been salivating for a chance; take into account that the accent he should in theory have been imitating was Almodóvar’s own, so he had the model right there (the movie is barely-hidden autobiography). Critics almost ripped Cuban Jorge Perugorría apart for slipping once in Volavérunt, when actually they should have been chopping up Bigas Luna for getting him a “newscaster” accent coach rather than sending him to live in Saragossa for a few weeks and pick up Goya’s Aragonese (despite his efforts, Goya was never able to get rid of the accent).
¡Grathiash!
From what I’ve read, Brad Pitt nailed the “Pikey” accent in Snatch.
If you ever meet an Australian, try telling him/her that Meryl Streep can do accents.