The soundtrack to the Zoo TV TV show which debuted around 1995 on MTV for a brief late late night run. I believe there were rumors they were going to do music for it, but it never happened. (Though the Edge might have composed some background stuff -- unclear.)
There was talk of a follow-up to the Passengers album. Obviously that never happened.
The ZOO TV Sega Genesis game referenced by Bono in a '93 Rolling Stone interview where players could remix U2 songs.
A bass-powered track from the Hansa sessions that they were working on with Meshelle N'degeocello in 94, referenced in Flanagan's book.
The ZOO TV multicast, for which they had filmed all kinds of video projects.
"In Cold Blood" -- the Bono poem that was supposedly recorded over the Numb backing track, as well as the unreleased Zooropa sessions.
The Rick Rubin sessions from '06 or '07.