According to a study conducted by Slacker in 2007, about 70 percent of music enthusiasts don't want to spend hours creating the perfect playlists, meaning that unless you're a DJ--or just really hard-core about music arrangement--you'd probably prefer to just listen to your library on shuffle or have someone else do the mixing for you. Internet radio stations such as Pandora and Slacker, and social recommendation sites such as iLike and Last.fm already do a good job providing the latter.
However, if you're craving a mix for a specific mood or activity--or if you want to limit the DJ powers to a specific set of personal acquaintances--Songvote is the service for you. This extremely nifty (and completely free) Web-based app lets users create collaborative playlists based on a simple time-restrained voting system. Using it is ridiculously straightforward: visit the site; create a theme-, event-, or activity-based contest; and then sit back and wait for the votes to roll in. Once the time is up (based on an end date you specify), Songvote provides a final playlist based upon the results of the voting. Each mix also includes direct links to AmazonMP3 and iTunes to spur song purchasing.