NicoNicoDouga has a contract with JASRAC(Japanese Society for rights of authors, Composers and Publisher), and thus NicoNico users are legally allowed to post songs registered to JASRAC if they sing the songs with their own voices, and NicoNico will pay JASRAC which gets distributed back to the original creators of the songs. Uploading the songs from CDs etc is illegal though.
There are many people who uploaded their songs and here are some of the great ones introduced in this vid.
Here we go!
We Are the World- "Zenbu Ore" (solo part is sung by a single person!)
Get your headsets ready! "Comparison with the original"(left ear=original, right ear=zenbu ore)
SKILL "Zenbu Ore" by Yamadan
Get your headsets ready! "Comparison with the original"(left
ear=original, right ear=zenbu ore)
You Raise Me Up
Without You
Melt -Band Edition-
Last Night, Good Night
Rouningyou no Yakata
Chorus by one man. Ookina Furudokei
Chorus by one woman. NicoNicoRyuseigun
Blossom
Sweet Memories
Shinjitsu no Uta
Tamashiino Rufuran
Pollyanna
Cross Road
One More Time, One More Chance
Harukaze
Kaede
Mononokehime
Hello Windows
Shounenki
Celluloid
Letter Song
Aisuru Hito
WALK
Rain Stops, Good Bye
Bokuga Tsukiwo Miruto
Curtaincall
Sugar
Suimin Jikan
Gocchani!