Is there a karaoke trainer for OS X? is there an easy-to-use karaoke training software for Mac OS X?
My point is, I select the .kar file (or the vocals-only .mp3) and I sing against, then I listen to what I've actually sang?

FWIW, I'm no professional singer. I'm a software developer, but singing is my hobby, so I seek utility to help me train myself singing...
You could give UltraStar Deluxe a try:


  A free and open source karaoke game inspired by the Singstar™ game available on the Playstation®. It allows up to six players to sing along with music using microphones in order to score points, depending on the pitch of the voice and the rhythm of singing. 


The content is somewhat limited though.

P.S. I'm thinking many usefull information could find out here