Here's a duet that really works well - Pat Monahan (lead singer of Train) and Brandi Carlile. The song was on Pat Monahan's solo album, Last of Seven, which was released this past September.
I've never really been that big into Train, just listening to a couple of songs, but Monahan's voice is really good here, although my favorite of the two is by far Carlile. Their voices match extremely well for this matchup.
This song really works well as a duet - it's a love song, and these tend to work well, especially as a duet. Here, it works especially well with both a male and female singer.
Pirate on the Run - Pat Monahan and Brandi Carlile