If the past year has proved anything, it's that Travis Barker is the king of collaborations. From providing instrumentation and production for Machine Gun Kelly's Tickets To My Downfall to featuring under FEVER 333, Atreyu, Tyler Posey and phem, the blink-182 drummer has been keeping busy. But that's hardly a change of pace.

Barker has been extending his talents far and wide since the early 2000s. Between multiple side projects, including Transplants+44 and Box Car Racer, and genre-defying guest appearances, his discography is nothing short of impressive. It's so stacked, in fact, that it's almost impossible to commit it all to memory.

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Read on for 10 surprising collaborations you may not have known Barker did.

“Sour Apples” – The Cool Kids

Don't feel too bad if this one flew under your radar. The Cool Kids churn out so many collaborations that it can be hard to keep track. The hip-hop duo teamed up with Barker in 2011 on "Sour Apples" for When Fish Ride Bicycles. If the name sounds familiar but you don't recognize their catalog, that may be because they also featured on one of Barker's tracks, "Jump Down," earlier that year.

“Jump” – Flo Rida

No doubt, we all know Flo Rida's "Jump." It was only one of the most popular songs to come out of 2009 (and maybe even the decade). But did you realize that Nelly Furtado wasn't the only notable feature on the track? Barker received credits for not only contributing on the drum set but also for composition and production.

“Don’t Keep Me Waiting” – Britney Spears

Of all the tracks on the list, this one made our jaws drop the furthest. While Barker and Britney Spears were both notable 2000s staples, we can't say that we ever expected them to feature on the same track. However, the two came together in 2011 to record a bonus track, "Don't Keep Me Waiting," for the deluxe version of Spears' album Femme Fatale.

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“Gone In September” – Mike Posner

If it weren't for the album credits, we never would have guessed that Barker made a guest appearance on Mike Posner's 31 Minutes To Takeoff in 2010. His contributions in "Gone In September" sound so pared back and low key compared to the pop-punk energy that we best know him for. We'll just let that speak to his versatility, though.

“Stairway To Heaven” – Mary J. Blige (Led Zeppelin cover)

Cue the double take. Did you ever expect Barker to collaborate on a Led Zeppelin cover with iconic R&B artist Mary J. Blige? That's the exact type of genre-defying magic that 2010 brought us. The two teamed up to take on "Stairway To Heaven" along with Randy Jackson, Steve Vai and Orianthi.

“Trouble” – P!nk

At this point, we're starting to wonder if there were any 2000s hits that Barker didn't make a guest appearance on. Believe it or not, though, he contributed his percussion skills to P!nk's 2003 track, "Trouble." The song also featured a notable collaboration with Rancid vocalist Tim Armstrong, who Barker has played alongside under the supergroup Transplants since 2002.

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“Elephant” – Idiot Pilot

This one may be in a more familiar alternative realm than Spears or P!nk, but it doesn't prove any less surprising. While Idiot Pilot are a strong and persisting force in the scene, they don't boast the same mainstream status as many other bands that have collaborated with Barker. Still, the act managed to snag the drummer for a feature on "Elephant" in 2007.

"Bad Man" – Pitbull

A collaboration between PitbullRobin ThickeJoe Perry and Barker certainly sounds like some kind of MTV fever dream. But isn't that just representative of most of the late 2010s? The four teamed up for the track "Bad Man," which is housed on Pitbull's 2017 album, Climate Change. All we can say is that it's exactly what you might expect.

You’re Welcome – Cokie The Clown

Any fan of NOFX remembers Fat Mike's "Cokie The Clown" persona. The character first debuted under their 2009 EP of the same name before becoming the moniker of Fat Mike's solo venture a decade later. Did you know, though, that Cokie's pop-punk foundations aren't limited to the band? Barker contributed drums for the project's debut album, You're Welcome.

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The Best Damn Thing – Avril Lavigne

How could The Best Damn Thing get even better? By mixing in some Barker beats, of course. It's not exactly surprising that two of the most iconic pop-punkers came together in the 2000s. It is, however, totally underrecognized. Barker contributed to multiple songs on Avril Lavigne's renowned 2007 album. Though not credited outright on the tracks, his beats can be heard on "I Can Do Better," "Runaway," "I Don't Have To Try" and the bonus song "Alone."

Which tracks were you most surprised to learn that Travis Barker featured on? Let us know in the comments below!