Remember a few weeks ago when YouTube made a big deal about its new music video page that sorts out every single music video that's hosted on the service? You can now put it to far better use with a new Firefox extension called Scrolling Lyrics Player (SLP) that turns each video page into a honest to goodness karaoke player.
Once installed, SLP will sit just to the right of any YouTube video and do a search for the lyrics of whatever music video or song recording you're watching. It then syncs that up with the timing of the video, which I might add, is nowhere near to being an exact science.
What's nice is that you can adjust the start time of when the lyrics begin, so if you're watching something that does not time up to a song's lyrical timeline, it's as easy as scrolling your mouse wheel up or down to get it in sync. It also does not enable itself on what it perceives as a non-music video, leaving the normal YouTube UI intact.
SLP has been built using Karaobird. Songbird users will remember that name from the Songbird add-on which does the same thing when playing music tracks from within Songbird's music player.
In my brief time with this add on I ran into quite a few troubles with it recognizing obscure songs, but it did a great job with popular content. While it may not have any reduction technology to get rid of existing vocal tracks, or per word following like you'd get with proper karaoke software, it's great if you want to turn your laptop or media center PC into a makeshift party machine.