compuhost, the only choice. It has an "a/b deck" that will automatically flow from one song to the next inbetween karaoke. Hit "GO" and the filler fades, and goes right into the karaoke track. When the karaoke is done, it automatically goes right back to music.
It automatically keeps track of your rotation too, and has a scroll bar at the bottom of the screen so everyone can see who's up next. Current singer is automatically announced as "now appearing"
You can keep track of multiple song submissions and duets, as well as singer's histories... no more song slips to keep track of!
Just google "compuhost" and take the first link.
I've been using it for two years now, and everytime another dj/kj sees this program thier jaws drop from envy... Trust me, there is no program finer.