This is one of my favorite cuts from Ben Folds Presents University A Cappella, "Not The Same." Before this I didn't listen to much a cappella, but this album has really been a mind-blower in terms of the range and dynamism of the human voice. Amazing stuff, and one of my unexpected favorites of this year. Here is the song.
Speaking of all-vocal songs, here's an old favorite, "I Didn't Understand," by Elliott Smith.
Oh, uh. On XO it's all multiple track vocal harmonies, but there's no video for that. This is ES playing it on piano. It's every bit as beautiful, though. Also, the video's a bit out of sync with the audio.