Roberto benigni won the Oscar while being nominated for both writing and directing. That's quite unique.
I know Keith Carradine won the Best Song Oscar, though he hasn't been nominated for acting. But it's an achievement in itself seeing how song writing isn't what he's usually known for.
There are many actors who get nominated for doing short films. Christine Lahti won the Oscar for making a short film, and she was also nominated as an actress for Swing Shift.
And Brad Pitt won the Best Picture Oscar for 12 Years A Slave.
I know none of this would qualify for your list but I think it's quite impressing to see actors win or get nominated for doing something else besides acting.

So this question stayed on my mind enough that one night (this very night, in fact) I decided to look it up to find the actual answer. I clicked on each and every IMDb listing for everyone who has won an acting Oscar to see if any of them had more than just the acting Oscars. And I can confirm that the four that have come up in this discussion are, indeed, the only four people to have Oscars for acting as well as in a category other than acting! Here are those four again:

• Barbra Streisand (Best Actress 1968, Best Original Song 1976)
• Michael Douglas (Best Picture 1975, Best Actor 1987)
• Emma Thompson (Best Actress 1992, Best Adapted Screenplay 1995)
• George Clooney (Best Supporting Actor 2005, Best Picture 2012)
  And again, I only counted Oscar wins in competitive categories. So there you have it!

Oh, and hey I just realized another very close one. Roberto Benigni won Best Actor for Life is Beautiful. He also wrote and directed that film, which won the Oscar for Best Foreign Language Film. Technically, the winner of the Best Foreign Language Film award is "Italy"; the director traditionally accepts the award on behalf of the winning country, but is not officially an Oscar winner. If you're not being really strict with the rules, though, you might consider Benigni as part of the elite club of people who have won Oscars for acting and something other than acting.
I'm more interested in the official rules though!

A couple of people have cited Laurence Olivier as having an Oscar for producing Hamlet as well as his acting Oscar for that movie. However, IMDb doesn't appear to have producer credit for him for that movie, and it says he has just won the one Oscar for Best Actor. Anyone have any more intel?
Also someone mentioned Lee Grant as having an Oscar for Best Documentary. Again, IMDb only shows her having the one Oscar as Best Supporting Actress. Can someone elaborate on the situation with her potential Documentary award? Which documentary is it?
Someone also mentioned Ben Affleck, but he doesn't have an acting Oscar. He has one for writing and one for producing, a feat shared by the likes of James L. Brooks and the Coen brothers. However, the original question was about people who have Oscars for acting as well as something other than acting.
Someone pointed out that Matt Damon might eventually fall into this category; I agree and hope he eventually does! I personally don't think Ben Affleck is a strong enough actor to be a likely candidate for this. He has no acting nominations to date. (Uh-oh, have I stirred up another fierce opinion debate? ;))

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Laurence Olivier won for acting and producing Hamlet and Lee Grant won for acting for Shampoo and won for best documentary as well.

Ben Affleck would qualify too
Best Picture 2013, Argo
Best Writing Screenplay 1997, Good Will Hunting
Rush

Star Wars
(1977) holds the record for most Oscars without winning Best Picture or Best Director (6).
"Everyone is ignorant, only on different subjects". Will Rogers (1879-1935)

Really?! That's interesting and impressive, thanks!

Thanks! I love facts like that! Related: I believe Fosse's Cabaret is the film that has won the most Oscars without winning Best Picture.

Thanks for collecting the information and putting this list together. I've always wondered about the unique achievements of actors/actresses. I'm a little obsessed with the multi-talented and triple threats of the industry.
Some other unique achievements: Bob Fosse was the first to win an Academy Award, a Tony, and an Emmy in 1973. He's also the only person to win all three awards for category of Best Director.

Hey, thanks for the answers & discussion on here! Since I posted originally, George Clooney has joined this elite club, having won the 2012 Best Picture Oscar for co-producing Argo, as well as 2005 Best Supporting Actor. So It appears we have a list of 4:

• Barbra Streisand (Best Actress 1968, Best Original Song 1976)
• Michael Douglas (Best Picture 1975, Best Actor 1987)
• Emma Thompson (Best Actress 1992, Best Adapted Screenplay 1995)
• George Clooney (Best Supporting Actor 2005, Best Picture 2012)
  Interesting discussion about Emma Thompson deserving her Oscars too! I stand by my opinion that Mary McDonnell was more deserving in 1992. However, I love Emma Thompson as an actress overall. She has shown a great deal of versatility throughout her career. I personally was not as moved by her performance in Howard's End; I found it authentic but a teeny bit dull. On the other hand I was very moved by her performance in The Remains Of The Day. And Sense And Sensibility was beautifully written.

Streisand also won one for Best Actress (but she tied that year with Katharine Hepburn) and another one for song-writing for "Evergreen", which she shared with Paul Williams (I think that is his name.the short blond songwriter).

Clint Eastwood never won for acting which is what the original poster asked. He won in 2 categories, but acting wasn't one of them. Laurence Olivier won for acting and producing Hamlet.

Clint Eastwood.

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UHM.yesshe DID ACTUALLY RE-WRITE MOST OF THE DIALOGTHEREBY ADAPTING IT TO FILM.
You know..you're clearly pathetically ignorant. You're a waste of my time. You have nothing to say. All you say is that I "say nothing", when in fact, you're talking about yourself. You voice opinions, which are finethey're like beep has one. But when you consistently spout nothing but misinterpretations, lies, bad taste, and the worst critical assessment of what great talent is, and great storytelling, you become quite, quite tiresome. And you just hold up a magnifying glass through which all the world (well, readers of your little posts) can witness the great magnitude of your sheer stupidity.

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"Thompson did not deserve either of her oscars - certainly not the acting one. The screenplay win was more justifiable, I suppose; but I think Tim Robbins should have won for Dead Man Walking - and if he had then he would now be the only perosn to win oscars for acting and writing and not her."
God, this assessment couldn't be further off the mark. First of all, Emma was in virtually every scene of the long, brilliant film "Howards End". Her character makes a solid arc. The profound dialog and storyline were BEAUTIFULLY executed by Ms. Thompsonshe was pitch perfect. I know she was up from stiff competition, in terms of notable actresses, but her performane far outshined all her contenders that year.
As for the writing.you try converting Jane Austen into a modern-day, digestible movie. Many have been done since, but Emma's adaptation of Jane Austen was one of the first, at the time. Her mastery of Austen, her characters, the dialog, everything, was sublime. She had rich characters, great dialog, and great actors interpreting her lines. No, I think she more than deserved her Oscar for that one.

You obviously didn't understand one minute of Howards End so why bother writing about it?

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You've got to be kidding to say that she was miscast. Universal acclaim, every major award won, a career-making and movie-making performance. She was scene-perfect in that film playing a "normal" woman, i.e. not a Prime Minister, Serial Killer, Queen, Psycho ballerina, etc. This was an extremely challenging role to get the nuances of that character and that story. If you think she was miscast and "thoroughly average" then you know nothing about movies, acting or art.

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