Extra Fun:

SoundFlower - is a free app for Mac which can wire you up between GarageBand and/or any other audio program or mixer, and the karaoke site - meaning you can add reverb, compression, EQ or other vocal effects as you record.
http://www.macupdate.com/app/mac/14067/soundflower

Not for Windows. I don't know of an app that does this for Windows, but you could check into what Audacity does, or if your recording software can export to a Flash player - this is usually what will do the trick.

ManyCam - I know some of you know this one already, from Skype or other video apps! It will add video effects to your webcam (if you have one) and interact with the karaoke site; so you can use a still image, give yourself a pair of funny sunglasses or fun frame around the video, or use your own prerecorded video to stream along with the song instead of live vid.
http://www.manycam.com/

Windows OR Mac downloads - free.

Questions getting anything set up? Confused out at the new websites?
Post here!

I can take and post detailed screenshots if necessary. LMK.

Thanks!