Spotify on Thursday unveiled a long list of new music discovery features, including options to follow big-name tastemakers and your online friends. And, in a surprise move, Spotify also signed on Metallica, the metal band that famously sued Napster in 2000.

As The Clash's "Rock the Casbah" played, Spotify CEO Daniel Ek took the stage to unveil the music-sharing platform's new personalization options. A new "Follow" tab lets users check out recommendations from friends on Facebook (FB), Twitter and Tumblr. They can also choose to follow other "influencers," like musical acts and media outlets.

The "Discover" tab will include all of those recommendations, plus suggestions based on listening history and new releases from artists whom the user follows. Users can save songs and albums to their "Collection" for listening later. The new features will be available on Spotify's desktop client over the next few weeks, and will come to both the web and mobile platforms soon after the new year.

"This means everyone's Spotify is going to look different," Ek said. "If you're a metal fan, Spotify will be all about metal."

Ek showed off the new Discover page, which will surface related music below the song the user is currently playing. They can choose to play it, save it for later, or skip to a new suggestion. An audio preview feature lets users jump from playing one song to checking out a short clip of a recommendation. They can then automatically leap back to where they left off in the original song.

When users choose to play a new song, the related artists/music page automatically refreshes.

The goal is "to keep people going down that path and trying new music," Ek said.

Ek also revealed a few stats. Spotify now has more than 20 million active users, and 5 million are paid subscribers. Of those paying customers, 1 million are in the U.S. Spotify hit American shores in July 2011.

Ek ended the feature reveal with a perk for artists: They'll be able to send push notifications to users' mobile phones when they release new music. Ek showed off a message popping up on his Apple (AAPL) iPhone.

After that demo, things got weird.

First, Napster co-founder and former Facebook president Sean "You-Know-What's-Cool-A-Billion-Dollars" Parker walked onto the stage. Moments later, Metallica drummer Lars Ulrich joined him. Parker and Ulrich hugged -- shocking many of those in attendance.

In Napster's heyday, Ulrich was the most outspoken critic of both Napster specifically and of illegal file-sharing in general. Metallica ended up suing Napster in 2000, in a landmark case that ultimately led to Napster's demise.

The point of Ulrich's appearance was that Metallica, which regained control of its own catalog on November 30, will now makes its music available on Spotify.

Parker, a Spotify investor and board member, spent a while waxing poetic with Ulrich about their newfound friendship. The pair chalked up their past disagreements to both miscommunication and media hype.

"We wanted music to be free as in freedom, not for free," said Parker, who blamed the press for twisting Napster's mission.

Ulrich said the misunderstanding between Metallica and Napster was "about control, not money." (That drew a catcall from the audience: "You sued your fans, Lars!")

Parker added that if "we'd just sat down and talked about what our intentions were," the lawsuit might not have happened at all.

Metallica's change of heart developed over several years, Ulrich said. He blamed the band's former reticence on both "being younger" and not fully understanding Napster's point of view. He liked Spotify's music streaming approach from the start, he claimed.

Metallica famously declared that its attitude toward Napster was "you f--- with us, we'll f--- with you."

Those profane feelings appear to be long behind Parker and Ulrich, who ended their onstage chat with yet another hug.