On my occasional visits to the site Rap Genius, I have thought this is so cool, and why doesn't this exist for other genres of music?  And why not other genres of content period?  Well, I don't know if the latter one was an idea that I had, but it turns out it was somebody's idea, certainly http://ow.ly/eWHkI

"Andreessen noted that when he and Eric Bina first built Mosaic, they initially built a 'group annotation' feature directly into the browser but later dropped it because their implementation required a server to host all of the annotation.

'I often wonder how the Internet would have turned out differently if users had been able to annotate everything — to add new layers of knowledge to all knowledge, on and on, ad infinitum,' he wrote. 'And so, 20 years later, Rap Genius finally gives us the opportunity to find out.'"
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