These artists have some truly incredible harmony vocals.
Firstly, Gillian Welch and her long=term partner David Rawlings, an American folk due specializing in sparse, haunting music. They worked on the soundtrack for O Brother Where Art Thou. Gillian voices one of the sirens. She is one of my favorite songwriters, and he is one of my favorite guitar players. Every one of their albums is excellent, but The Harrow and the Harvest and Time (The Revelator) are particularly fantastic.
This one has a wonderfully strange harmony at 2:19:
And this is just my favorite song by them:
And then there's Ye Vagabonds, an Irish folk due consisting of two brothers. The way their voices blend is stunning, and while they do a lot of traditional songs, they write some incredible tunes as well, like this one:
And this one:
And here's Lowlands of Holland, a traditional song: