Rap Genius is a website where users annotate the lyrics of popular rap songs—offering listeners insights into the slang and hidden meanings of, say, Kanye West’s latest, Clique. It is a popular online diversion, and in the eyes of some Silicon Valley venture capitalists, the web’s next big thing. Last week, Andreessen Horowitz, a firm that has backed Twitter, Facebook, Groupon and Skype, put $15 million into Rap Genius. Explaining the investment, partner Marc Andreessen said that he sees Rap Genius expanding beyond rap to other genres and art forms, with an online community annotating everything from literature to political speeches, scientific papers and the Bible. “The big missing feature from the web browser,” he wrote on the site, “is the ability to annotate any page on the Internet with commentary and additional information.” One useful note on this point: Andreessen speaks with some authority—he happens to be one of the creators of the very first web browser, Mosaic.