The Arcade Fire: Neighborhood #1 (Tunnels)
At rateyourmusic.com the album Funeral ranks as the second best album of 2004. So far I only know this one song by the band, but they definitely sound like they are worth checking out.
New album added to my CD-Collection:
Elliott Smith: From A Basement On The Hill
I won't make any "best-of-2004" lists, but it's no secret that (so far since I haven't heard them all) I'd pick American Idiot by Green Day as the album of the year. Best single is (probably) Gravity by Embrace.