Virtual Orchestra Studio (VOS) is a musical DJ game created by
the Koreans. If you like guitar hero/dream cast/korg kinda games,
you'd definitely love VOS (VOS is the keyboard version of those
games... Keyboard as in musical instrutment keyboard... of course
it's also played with your computer keyboard lah 
)
It's free to download from http://vos.hlwong.com/
And there're many songs to download too!
Some screenshots:
