[pirroh]

Google built something similar years ago--not a direct competitor to RapGenius, but rather a generic annotation platform for the Web.

Guess what? It's now discontinued: http://www.google.com/sidewiki/ It's not trivial to build annotation platforms that are both compelling and easy to use. Hat tip to RapGenius!

[streptomycin]

It is trivial to build a lyrics site, and it would be very helpful to search users who so often search for lyrics and wind up on shitty websites.