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):
Mass-tagging using VocaDB: https://rentry.co/vocadbtagging
Essential Vocaloid Crossfades
PDAFT for dummies (no longer actively maintained due to megamix officially releasing on PC):
MegaMix+ Modding Guide:
MegaMix+ Mods (more mods can be found in the Project Diva Modding Discord server):
Not sure what to listen to? Try something from here: