One Genre Savvy journalist even went so far as to say that Nielsen's job description was unchanged for his early dramatic roles and the comedic roles which he became better known for: both hinged on his ability to say completely ridiculous lines with a straight face.
Marx Brothers: Margaret Dumont, the classic foil for the Marx Brothers, especially Groucho. Zeppo Marx himself was a straight man for his brothers, as he had a non-comedic stage persona, but all the requisite timing and skill. After Zeppo's departure, other actors (such as Allan Jones in A Night At The Opera and A Day At The Races) would take the role.
Will Turner plays the Straight Man to Jack Sparrow.
More appropriately, Will is the Only Sane Man to a crew of pirates including: Jack; a highly superstitious and heavy-drinking ex-Royal Navy seaman (Gibbs); a dwarf (Marty); a pirate whose tongue was cut out and can only talk through a parrot (Cotton); the parrot (Mr. Cotton's Parrot, whom no one can quite figure out how he was taught to speak); two formerly cursed and none-too-bright ex-crewmen of Jack's treacherous first mate (Pintel and Ragetti); said treacherous first mate (Barbossa); an undead monkey; and a rather deranged voodoo priestess and imprisoned sea goddess (Tia Dalma).
Larry, since his style of humor was often to merely react to the extreme characterizations of Moe and Curly (and later Shemp).
It is probably more accurate to say that the miscellaneous guest actors in the shorts constituted the straight men, who (like the supporting characters in Marx Bros. films) are there to act appalled at the comedians' antics.
Withnail & I: In a less obviously comedic example, Paul McGann as Marwood/"I" plays a pitch-perfect straight man to Richard E. Grant (Withnail), grounding his manic performance in a solid emotional reality; the film wouldn't work nearly as well as it does without him.