Birds In the Air (what came after Colossal)
Orwell (members of Gainer, Gauge, Radio Flyer, Braid, Sky Corvair)
My Heart to Joy's last record 
Postal Service 
whatever Grown Ups had ready to record before pulling the plug
I'm pretty sure they said a few years ago they were working on something, and I think they had stuff back in the day, too. I know they want to follow Loveless up with something meeting or raising that bar, but that's a dang high bar; just getting close would be good enough for me.
U2 - Songs of Ascent / Danger Mouse sessions / dance album
Songs of Ascent was supposed to be ready w/in a year of No Line on the Horizon and contain stuff from the same sessions. That album was weak, but the extra material still sounded tempting. If the material from that abandoned material was supposed to resurface in the sessions with Danger Mouse, and/or be the same material they played live a couple years ago, I'd like to hear it.
More so, I want that dang dance album they've talked about to come together already. They were supposed to be working with will.i.am and RedOne on this material. As weird as it may sound, I seriously think they have a good dance album in them.
- Weezer/ Rivers Cuomo's Songs From the Black Hole
- Blink's initial follow-up to their Untitled album, before their hiatus.
- Pixies new stuff, which I still have hope for being released
- The last stuff by Nirvana, which I'm pretty sure isn't all released yet.
This thread is kinda depressing, haha. But a good one.
I really wanted to hear Far-Less's last EP before they broke up. They did release both their demo called Green and one called New Song, but that's it. They stopped updating their blog after saying it was going to be longer than originally planned. Also, I don't remember if it was gonna be the same EP, but I remember them saying there was going to be an EP of reworked older songs. Has there been any mention of what happened?
I also would've liked to hear Terminal's second album, My Chem's unreleased album(s?) and Box Car Racer's second album.
Edit: I did some Google searching, and found out they released the demos a few months ago. It's not everything they talked about, though.
I remember hearing a little while ago that Lou Reed and Metallica were collaborating on some music, but I haven't heard anything about it since then. I can only assume that the album has been scrapped, but I'm sure it would have been excellent.