"and not have been packaged in a single jewel case" I was thinking of Zappa's "Lšther" right before I read that.
Andy Partridge - Fuzzy Warbles (9 Discs)
Andy Partridge is one of my favourite songwriters. So a massive set full of neat demos and in-process versions of XTC songs would be great enough, but this is also full of unreleased songs, outtakes, and neat experiments. This thing is absolutely loaded with hidden gems.
Jellyfish - Fan Club (4 discs)
4 discs entirely of unreleased material for a band that only ever released 2 albums is almost a ridiculous notion but this boxset does everything right. I'm a huge Jellyfish fan and this makes getting a hold of all their non-album work a breeze cus everything is here. There's demos for both of the band's albums. live material from both of the band's tours showcasing one version of almost every song the band ever played live, including all the songs that they played live but never recorded. All the B-sides. and weird oddities, like the song they wrote for nintendo and the songs they wrote for Ringo Starr.
Beach Boys - Smile Sessions (5 discs)
Doesn't need too much explanation. Both an incredible reconstruction of one of the greatest unfinished albums ever made and a comprehensive documentation of it's fascinating creation.
Genesis - Archive volume 1 and 2 (4 and 3 discs)
All live, unreleased, and non album tracks. It's missing a couple things which is annoying, but it's not a deal breaker. Volume 1 is full of their live peak and volume 2 has a lot of really interesting b-sides many of which are better than the album tracks. Definitely NOT the 3 new boxsets tho cus the remixes are ass.
Motown Anthems - Various (4 discs)
No idea how well known this is, but I stumbled across it and it's great. Basically a comp of every classic Motown hit ever plus most of the lesser hits too. Put this on shuffle and have a good time.