"Such Great Heights" (The Postal Service)

The Postal Service's Give Up (2003) was such an unlikely record that came together in an unlikely way.  Dntel's Jimmy Tamborello just happened to meet Death Cab for Cutie's Ben Gibbard through a mutual friend; Tamborello asked Gibbard to contribute vocals to what became the Dntel track "(This Is) The Dream of Evan and Chan" for 2001's Life is Full of Possibilities; and they found the process of working together so effortless that Gibbard suggested they collaborate again in the future.
So between working on new Dntel music, Tamborello (who's based in L.A.) began creating backing tracks and sending them to Gibbard (who's based in Seattle) on CD via the U.S. Mail.  And when Gibbard wasn't busy working on music for Death Cab for Cutie, he'd take a few days to absorb the backing tracks, write lyrics, and then cut vocals/make edits, eventually mailing his contributions back to Tamborello.
In the 2013 documentary Some Idealistic Future, which commemorates the 10 year anniversary of Give Up's release, Tamborello describes the ease with which the project came together over the course of 2002.
"We really didn't send stuff back and forth very many times.  I'd send him the basic idea, and then he'd cut little bits out or move parts around or repeat parts that he wanted to sing on, and then send me back a guide track with his vocals."
Such was the case with the track "Such Great Heights."  One of the last tracks written and recorded for the album, the song arguably is the big "hit" from Give Up (even though it only topped out at #114 on the Billboard charts).  Thing is, it sounds like the kind of tune they would have pored over for hundreds of hours, getting every harmony, guitar lick, and synth blip just right before calling it "done."  So it's amazing to me that they created this intelligent, cliché-avoiding love song with its Lennon-McCartney-worthy melody by casually swapping ideas via snail mail.  For that reason alone, it's one of the most noteworthy singles of the 2000s.  But I'd also go so far as to call it the best pop song of that decade.