Similar Artists/Songs?

What are some songs that sound like these ones?
especially the duet ones.
Song 1:

http://www.youtube.com/watch?v=Nemst-oB3Po

Song 2:

http://www.youtube.com/watch?v=UskSU5BoyZs

Song 3:

http://www.youtube.com/watch?v=bRLCSxSR39s