I recently found a website with a number of Boris albums that are out-of-print or were never released in the US. Yeah!
Demo Vol. 3 EP (1994). With "Scar Box," "Mosquito," "Matozoa," and "Deep Sucker." Biiiig Melvins influence.
More Echoes, Touching Air Landscape (Boris and Choukoko No Niwa, 1999). A split with two very long songs. The Boris one is a 26-odd minute long drone. The Choukoko No Niwa track is a 24-minute Boredoms-like psych rock freakout.
Megatone (Boris and Merzbow, 2002). Like drone? Like Merzbow? Like Merzbow droning? This is that.
Archives: Volume One: Live 96-98, Volume Two: Drumless Shows, Volume Three: Two Long Songs (all three released 2005). Three discs! The first one has eight tracks of rocking Melvins-y Boris. The second, recorded in 1997, has three long drones. The third, recorded in 2001, has Boris's two long tracks of the time back-to-back. The recording of "Flood" is amazing.
04092001 (Boris and Merzbow, 2005). This one has five tracks from Heavy Rocks recorded live in 2001 with Merzbow doing his Merzbow thing.
Soundtrack from Film "Mabuta no Ura" (Essence Mix) (2006). This is the Brazilian version of an album I already have. Many of the songs are the same as on the other version, but the sequence is a little different, and the new tracks are pretty great.
Long Hair and Tights (with Doomriders, 2007). This is a Pink-era live recording that is split with the Massachusetts band Doomriders. I think I would like Doomriders much more if they weren't getting blown off the stage by Boris.
She's So Heavy EP (Ai Aso/Wata, 2007). This is a split 7" with Japanese pop star Ai Aso covering King Crimson's "Islands" on one side and a great song by Boris guitarist Wata (fronting a band that is, basically, Boris) on the other.
Walrus and Groon EP (Boris and Merzbow, 2007). Side A is a super-psychedelic cover of "I Am The Walrus." Side B shares a name with a King Crimson song and a different Merzbow song, but it is neither.
Cloud Chamber EP (with Michio Kurihara, 2008). Unlike their prior collaboration with Michio Kurihara, Cloud Chamber is a drone-based composition.