A bit of a clumsy title but just trying to emphasise that this doesn't include solo projects, or say collaborations like King Creosote/John Hopkins etc. Here's some of mine:
Arab Strap - first got interested in them after hearing Aidan Moffat's excellent contributions to the two Reindeer Section albums, Whodunnit and Nytol (Whodunnit - https://www.youtube.com/watch?v=MGRvaFarn-U).
Sparrow and the Workshop - The Fruit Tree Foundation charity project was very much driven by Emma Pollock and Rod Jones but offered up a good introduction to Jill O'Sullivan and ultimately her band (Dead Leaves and A Swollen Leg - https://www.youtube.com/watch?v=g-q06fynV7A).
Karine Polwart - Emma Pollock involved again, this time with Kenny Anderson and the Burns Unit project, which highlighted Karine Polwart's rather lovely voice (Blood, Ice and Ashes - https://www.youtube.com/watch?v=fvk6xmuQuRg)
Aspects - Bristol based hip-hop group cropped up on for a feature spot on a DJ Format album, while I'm a little loathe to start including hip-hop in this thread as guest spots as a way of discovery are so common seems a bit churlish to dismiss a genre. Any group who reference James Last and Geoff Capes in the same song deserve a listen (Charity Shop Soundclash - https://www.youtube.com/watch?v=Gzf4Of2Hdng)
So which artists did you first discover outside of their main job, whether it's an appearance with better known artists or a big name making a telling contribution to a bigger project..