Linux: We've featured an open source SingStar-like karaoke game before, but if you're looking for something a bit simpler, OSD-Lyrics automatically downloads and displays lyrics for a ton of popular Linux media players.
If you've been looking for other alternate uses for your Linux-based HTPC, you can check "karoke machine" off the list by installing OSD-Lyrics. Unlike previously mentioned UltraStar, it's not a full-fledged game; instead, it just displays lyrics for a number of different media players like Amarok, Banshee, Exaile, Rhythmbox and Songbird (to name just a few).
Surprisingly, it works very well. It downloads and displays all the lyrics automatically and somehow magically knows where each line begins and ends in a song, so you have to do little, if any, work to get it up and running. Just start up the application, play a song and go to town. If you want a more genuine karaoke experience, you'll have to seek out some vocal-less tunes yourself, although you can always just hook up a microphone and sing along with the full versions of songs you already have.
OSD-Lyrics is a free download for Linux machines.