The main (re)mixes I currently have:
Garbage - Who's gonna ride your wild horses (Bono on Vocal)
U2/Depeche Mode switching singers on So Cruel (Bono starts with Depeche, then U2 with Dave Gahan takes over)
Nine inch Nails - Zoo Station (Trent/Bono on Vocals)
U2 - Ultraviolet 1 (Hansa demo with parts of Spinning Head, The Fly, UV Light my way and one "easter egg" weaved in)
U2 - Zoo Station as it could have been (mix of Sky Down, Baby version, and the Eno "Bottoms" remix)
U2 vs. A-ha (The sun always shines on tv mixed with Beatiful Day)
Nine inch Nails - Hurt (NiN version with Johnny Cash on Vocal)
Johnny Cash version of "One" (with Bono on vocals), and the U2 version (with Cash on vocals)
I have more, but these are actually "listenable" at the moment

Thought to start off with a new (re)mix of Johnny Cash joining U2 on "One". It's Cash and Bono altering vocals and Cash's guitar joining U2's guitar intro: