This phenomenon is kind of really hard to explain to most non-Japanese readers I guess, but check these videos out. Among lots of songs, these two are my favorite.
The vocalist (Hatsune Miku) doesn't really exist. It's a software (a.k.a Vocaloid) that you can program to let her sing whatever song you want. Lots of users (I guess mostly not professional) use their Desktop Music composing software to let Hatsune Miku sing their original songs and they are uploaded to Nico Nico Douga (Great video sharing site in Japan. Basically like YouTube with time-sync floating comments, but more features and culture there, enough to make me addicted... maybe another time to explain) and YouTube. Actually most songs come with an original Japanese lyrics that describes and represents the characterics of Hatsune Miku, which is so awesome and touching as well.
It really is revolutionary I think...
Learn more about Hatsune Miku on the following links, if you're interested: