New Order's Substance. Moby's albums Play and 18 came in special editions with b-side discs that are hard to find, but the songs are stunning. More simple, but just as beautiful. U2's Best of 1990-2000 special edition with b-sides is mostly mixes, but they're all pretty good. The Salome mix in particular is the danciest thing they've ever done. And The Killers Sawdust.
I've got that one too! It's not brilliant, but worth having. I don't even have many regular OMD albums: only Dazzle Ships, Best of OMD (that one is awesome though!), Sugar Tax and Universal. Maybe unusual to get a B-side compilation without having everything else...
(Of course I have listened to most of it, you know what I mean...)