Rank'em distinguishes itself from other music-ranking sites by focusing on individual artists' catalogues. You can't rank and compare Rush against Yes, but you can look at and rank their individual discographies against themselves.
Most music services have you giving thumbs up or thumbs down to songs or artists in aggregate against the entire catalogue of the site. Rank'em is a site for dedicated fans, fans who don't just say Roundabout by Yes is a pretty good song, but would consider ranking the live performances at the House of Blues and Montreux over other releases of the song. Since you're not ranking one band against another but instead the discography of a band against itself, Rank'em provides an interesting look into the musical history of an artist or band and how well received their song catalogue was among fans.
Rank'em is free to use, but participating in the ranking process requires basic email registration. Have a favourite site for creating or viewing music-related rankings? Let's hear about it in the comments.