YouTube & NND
I have a really strong fondness for YouTube, and especially NicoNicoDouga. When I was a child NND was actually my primary source of entertainment, as I wasn't technically allowed to be on YouTube. As you can imagine though, that means I had a very unique upbringing. Regardless, these are embeds of a few YouTube or NND videos (some of which I need to or want to archive) as well as channels that I adore. Don't take this as a list of reccomendations, but if you really want to you can.
Kumikyoku Nico Nico Douga | 組曲『ニコニコ動画
There is more love than I'm willing to admit for the KNND, particularly because it was one of the first songs that I learned to sing and can read in Japanese. I remember when I heard about the 10th anniversary video and I was so confused that people were still doing Medleys. When I popped on, and started browsing around I genuinely cried at the nostalgia. This was actually one of the most striking pains of memory that reminded me of who I was and left me in a position of wanting to recover myself again. I still know the lyrics to this day.
Nightcore
You probably figured this much, but Nightcore was the next safe bet. Easily obtainable without YouTube being the main supplier, due to several websites that offered music downloading. Heck, I even remember an old 8tracks. I miss those days, I loved making mixes for my friends and sending them like mixtapes over the internet. I still do that now, but it's YouTube playlists at best. But I remember Nightcore fondly, as I would often fall asleep to this music. It was also what introduced me into Electronic music, which is where I fell in love with most of who I listen to today.