One of the things I love about Christina is that she loves collaborating with other artists and has been doing so since her debut. Here are some of my favorites collaborations (in no particular order) that she made outside of her albums, with some tidbits about them.
Nobody Wants to Be Lonely with Ricky Martin
Fun fact: The song was nominated at the 44th Grammy Awards in the category of Best Pop Collaboration with Vocals but lost out to another one of Xtina's collabos, "Lady Marmalade".
Lady Marmalade with Lil' Kim, Missy Elliot, Mya and Pink
Fun fact: Christina stated that the thing she remembers most fondly is the last scene of the video: “What I remember best is standing on the stage for that end shot where we’re all marching out in unison together, all singing our parts and belting our asses off, and it just felt so cool,” Aguilera concludes. “It was an unforgettable moment, it just felt like it was meant to be. Sometimes you have these things in life where it just felt like the right time and the right place, and this was definitely one of those great moments in my career.” (Source)
Si No Te Hubiera Conocido with Luis Fonsi
Fun fact: The song didn't get a video, but it was a big ballad at the time in Latin America. Fonsi recalls: "She’s amazing. That was awhile back. That was like 1999 or something like that. We were both starting out and she was putting out her first Spanish album. I got to work with her in the studio and see her sing in front of the mic, which was awesome. She’s great. One of the best voices out there still to this day." (Source
Car Wash with Missy Elliott
Fun fact: The standard music budget for a film is between 2-3% of the total budget, although this figure can vary depending on the production. For an original song by a well-known artist, the budgets can approach sky-high numbers. Christina was paid $1 million for a single song in Shark Tale...No wonder she loves soundtracks so much, she was and is that bitch. (Source)
Castle Walls with T.I.
Fun fact:According to producer Alex Da Kid, it was supposed to be released as a single but it wasn't, probably due to T.I.'s incarceration. (Source)
Do What U Want with Lady Gaga
Fun fact: Lady Gaga released four remixes of this version with Xtina instead of R-Kelly's. (Source)
Feel This Moment with Pitbull
Fun fact: Christina and Pitbull performed this song live at the 2013 Billboard Music awards and were joined by Morten Harket from A-ha, whose members had originally written and composed "Take On Me", the song that FTM samples.
Moves Like Jagger with Maroon 5
Fun fact:The video features Adam Levine and several extras sporting a "Who the f--- is Mick Jagger?" t-shirt at various moments in the clip as a lighthearted way to make fun of the new generations not knowing who Mick Jagger is.
Tilt Ya Head Back with Nelly
Fun fact: Both Janet Jackson and Britney Spears were considered for the song, but Christina was the one who got it. (Source)
Like A Virgin/Hollywood Remix with Madonna, Missy Elliott and Britney Spears
Fun fact: It was rumored that the initial lineup for this performance was Pink and Jennifer Lopez instead of Christina and Britney.
Tell Me with Diddy
Fun fact: Apparently the track was initially created for Diddy’s protegés Danity Kane. But he decided to keep it and added Christina to the mix.
Say Something with A Great Big World
Fun fact: Say Something was initially created and recorded by the duo without Christina, and it went largely unnoticed until she saw it and decided to record her own version with them. "'Say Something' was danced to on 'So You Think You Can Dance' almost two months ago, and so many people responded to it," Axel told Billboard. "In that whole process, someone on our team played it for someone on Christina's team, and we got a call that Christina wanted to record it, and then, literally a week later, we were in L.A. recording it with her." (Source)
There are other collaborations that I'm missing but these are the ones that I consider the most iconic. Feel free to post others you like!
Modnitta 
A Cock For You 
