vocadb.net - Comprehensive database of all Vocaloid music; songs, albums, producers, etc. Also includes links to purchasing songs/albums. 

https://www.nicovideo.jp/tag/Vocaloid_Hall_of_Legends - The most popular Vocaloid songs of all time. You're bound to find something good here.

https://vgperson.com/vocalhighlights.php - Monthly/Yearly Vocaloid highlights since 2011 with a random song selector.

Hazuki no Yume and others dump and other deleted stuff (removed from Youtube):
https://mega.nz/#F!I6BzSQQS!QhV_tSMGU2zrAjK6jzTjYA
https://mega.nz/#F!46IGHQZa!a1JYn3oPH_CytbNyJuLvDQ
https://mega.nz/file/oYhEgQwY#oSoyjljgqLcMdYCIx3rEluhF-MQfsnlvrYzvEU6Fgpw

Mass-tagging using VocaDB: https://rentry.co/vocadbtagging

Essential Vocaloid Crossfades
https://drive.google.com/drive/folders/0B6drFCl6uqFyfkhZcHJxN1F3RHZFajlkaVRfU05PdmJ0aWduTnpud3dqN3gxNUtFT3pMeUk?resourcekey=0-AD6u7CCzMGKGrMQRew8EPw&usp=sharing

PDAFT for dummies (no longer actively maintained due to megamix officially releasing on PC):
https://rentry.co/PDAFTforPC

MegaMix+ Modding Guide:
https://hackmd.io/@Still/mm-megathread

MegaMix+ Mods (more mods can be found in the Project Diva Modding Discord server):
https://gamebanana.com/games/16522

Not sure what to listen to? Try something from here:
https://pastebin.com/dMX8CJhk
https://i.imgur.com/FsgJ9iI.jpg
https://i.imgur.com/DT9wKOL.png