I really like the cover of Purple Haze they did for the "Stone Free" Hendrix tribute compilation. Was it Robert only? (It seems to me a thing he may have done on his own).
I really like "Yesterday's Gone" with Reeves Gabrels (back then not yet a member of The Cure).
Of course, Burn (to my knowledge, it was just Robert and Boris).
Pirate Ships' cover is another one I really like (again, a solitary endeavour of Robert).
Ah, well, back in the old days, the whole side project of The Glove with Severin: I love that album.
Some of these are not collaborations, in the sense of making music with other musicians outside of The Cure, but I included them in the sense that they were collaborations with other projects, beyond The Cure.
There's scope for some wide discussion. Some of the orphaned tracks are banging.
Oh, I forgot Robert's remix of Jewel, the Cranes song from the album "Forever".