I just dropped $15.84 on "Leaving Last Night," the soundtrack to a breakup between two people I will never know.
But here's what I do know, based on a note left by the playlist's anonymous creator in the iMix section of Apple's iTunes music service: "Summer. It's 3 a.m. on highway 94. The windows are open, and you're headed west. She's gone."
The drive begins with "Track 1," a postrock dirge by Iceland's Sigur Ros. Soon comes Death Cab for Cutie's "Passenger Seat" ("I roll the window down/and then begin to breathe"). By the time the 80-minute ride ends with Pearl Jam's "Yellow Ledbetter" ("And I know/I don't want to stay at all"), I'm perfectly miserable. As playlists go, it's a masterpiece.
I've become obsessed with iMixes, where users express themselves by sequencing and posting track recommendations. The often emotional themes include new romance, duty in Iraq, and other life milestones. "If I were to pass away," writes one iMixer, "I would want these songs to be played at my funeral."
With more than 800,000 playlists, iMix is the most recognized of a bunch of similar services emerging on the Web. The confluence of technology and emotion is creating a new generation of cultural tastemakers. And that's catching the attention of people like Mike McGuire, who, as vice president of research for Gartner Dataquest Research, tracks online music communities' growing clout.
McGuire and his research partner, Derek Slater of Harvard University, predict that recommendations by music consumers online will drive 25% of all Web-based music transactions by 2010, up from less than 10% today. Gartner expects the U.S. market for music downloads, excluding ring tones, will nearly quadruple, to $1.9 billion, in five years.
Playlist makers aren't professional DJs or major-label marketeers. Rather, they're people like Justine Saylors, an Oregon mom whose iMixes dedicated to her teenage son, Lance, have helped lift her from grief and raise awareness of the pediatric cancer that took his life. "I'm still getting a ton of letters re: the mixes," she wrote in a recent email exchange. Carla Wessel of St. Petersburg, Florida, posted her iMix, "Born to Be an Angel--In Memory of Nathaniel Joseph, 03/24/05," to honor her son who died at birth.
A diverse array of services are springing up to meet this demand. Goombah.com , for example, will analyze my music library and generate recommendations based on the listening habits of other Goombah members with similar tastes. Last.fm  turns my music collection into a personal Internet radio station where other members can tune in. Mog.com  provides a publishing space--a "mog," or music blog--where I can scribble about my latest discoveries.
What these sites share is a reliance on our democratic instinct to communicate and share. David Hyman, Mog's CEO, is passionate about that. "New technologies are starting to rattle the foundations of online self-expression," Hyman writes in his own mog. "Coupling these tools with technology to help you find people who share similar tastes will usher in a new wave of relationship building.... New communities will flourish and become the most powerful means of discovering what we want to consume."
Wessel, no foundation rattler, puts it in more human terms. "I made the list for myself initially, and then I discovered you could post them," she says. "I can't write a song, I can't write a poem, but this helps. And whenever I find something that has helped me, I share it with others."