Here are some nice songs to add to the vocaloid collection.

The Worst Carnival
http://www.youtube.com/watch?v=zC6c8rhxja8

Monochrome Dream Eater.
http://www.youtube.com/watch?v=_eNvLxMj0q8

The Game of Life
http://www.youtube.com/watch?v=OzHq2fDWH2I

Coffin of Sweet Death
http://www.youtube.com/watch?v=KFwV265PQCY

Enjoy!