Notation Software
 

Not sure if this is in the right section or not, hopefully it is though.

Is there a music notation software out there that will let you input a melody line or a few parts of a song, and it will generate the rest of the parts for the song?

That would require one hell of a musical database. So that won't be happening in the near future.
However, Finale is great if you just want to enter it yourself, it sounds the note you input so you know your doing right.
Also, the internet has an abundant supply of tabs, so you can input those in finale, and translate those into notes.
I use it to compose, and I love it.