GSnap - With GSnap you can get an auto-tune effect.
KaraFun - KaraFun is the best online karaoke. Over 35,000 karaoke songs, with studio quality, at home or on the go. Regular updates. Free demo!
Revoice Pro - Revoice Pro is an audio plugin used for vocal tuning, time correction and ADR.
Yokee - Karaoke to YouTube videos
CrispyTuner - Fully featured vocal tuning suite.
Sing! Karaoke by Smule - Karaoke with your favorite celebrity singers