This is a wide umbrella but there have been some interesting/odd collabertions over the years in the world of music. And by collaberations, it could mean working on an album or song together or performing together at a special event or whatever.
David Bowie/Bing Crosby -- They had that great collabertion on Little Drummer Boy/Peace on Earth back in the '70s.
Def Leppard/Taylor Swift -- Came across some YouTube videos of them, and while Swift doesn't have a power rock voice, her collaberation with Def Leppard on "Pour Some Sugar on Me" was decent. It was fun. There was an accoustic song that was better. All in all, I was impressed.
Martina McBride/Pat Benatar -- Saw McBride perform "Promises in the Dark" on some country music show a few years back, and Benatar came out and joined her on the song. Excellent job. McBride has a powerful voice that would work fine on rock songs.
Aerosmith/Britney Spears -- they did something together at the Super Bowl a few years back. Not good. Spears has her pop niche, which is fine, but her voice was definitely out of place.
What are some others?