Not sure if this counts, but I'm sure everyone has heard it. When Rainbow was first released, Ayu gave out a password to a special site that let you download a short instrumenal of Rainbow, before it came out and let you submit your own lyrics. I always considered it to be a sort of demo version of Rainbow. Sorry, no youtube link. :/
http://www.mediafire.com/?gqoldbherdxmkn1