GrokMusic is a music discovery site that will appeal to any music fan who's ever tried to describe a new band by naming other artists. As in, "St. Vincent? She's kind of like a guitar-playing Laurie Anderson crossed with early Genesis, with a little Steve Reich thrown in."
The site launched in 2008 with artist information and editorial content about music, then introduced a pair of free music-discovery tools in 2009. With MusicMap, you can enter any artist's name, and it will show you that artist's place on a visual map near related artists. Or, you can enter three artists' names into a Suggest Artists box and the site will return a list of up to 12 artists and ask you whether you like, dislike, or don't know them--GrokMusic also uses your entries in the Suggest Artists box to improve the correlations between artists. I found its suggestions to be surprisingly useful, particularly for obscure artists--the similarity engine seems to take popularity into account, so if you're already off into the weeds, GrokMusic will be happy to take you further. There were also a handful of what seemed to be mismatches, like Billy Joel appearing on my AC/DC map.
Once you've got your results, you can play several 30-second samples within the media player on the site, then follow links to purchase the songs from iTunes or Amazon MP3. You can also register to save your favorites.
Of course, many other music services have been doing this for a long time: Pandora is based on detecting musical similarities, and Apple built a similar Genius function into iTunes more than a year ago. Still, it's a fun diversion, and might come up with suggestions that these other services miss.