The number of non-US actors and particularly actresses doing excellent American accents on television and in films these days is surprisingly large. In addition to those already mentioned there’s also Poppy Montgomery (Unforgettable), Yvonne Strahovski (Chuck), Sophia Myles (Moonlight), Simon Baker (The Mentalist), Kevin McKidd (Grey’s Anatomy) and probably a bunch of others I’m forgetting. Cate Blanchett and Nicole Kidman have done good American accents. My favorite though is still Tracy Ullman. She has done American accents in film and on television. I was most impressed many years ago when she did a sketch on her show entirely in a spot-on Baltimore accent. I’ve never heard that done before or since by anyone, American or foreign.
It’s harder for Americans to judge how good foreign accents are being done, but Meryl Streep has never been criticized (as far as I know) for the many she’s attempted. Gwyneth Paltrow’s English accent is also quite good.
E.T.A. I was too slow on the Gwyneth Paltrow. I think she also did an English accent in Sliding Doors.