I skip the more known (and excellent) demos of bands like Emperor, Burzum, Behemoth etc and focus on more unknown bands, also I skipped professionally recorded studio demos. Most of these tapes are to find on youtube, I don't post any videos here, because this would mess up performance of the thread....
Nordic inlfuenced black metal:
Incarnator - Nordic Holocaust (1993) - close to Bathory, but still distinctive. No album followed after this.
Thorns - Grymyrk og Tröndertun (1991) - This is the band whom it is believed they developed the nordic black metal riffing
Baxaxaxa - Hellfire (1992) - Unknown masterpiece of early german black metal. No album followed after this.
Enkil - Self-titled (1995) - US raw black metal from the mid-90s in norse traditions. No full-length album followed after this.
Traditional german school black metal:
Graupel - Als der Nebel... (2001) - german school of black metal. one of the rawest stuff ever recorded
Grabgesang - Of Medieval Graveyard Forests (1995) - german raw black metal, primitive and raw, with some catchy melodies. No full-length album followed after this.
Pagan Winter - In the Shadowlands
Polish black metal:
Kataxu - North (1995) - Bizarre symphonic black metal, the terrible sound makes it even more fascinating
Profanum - Under a Black Wings of Emperor (1994) - raw black metal with decent usage of keyboards
Gontyna Kry - Welowie (1997) - early polish atmospheric black metal
Thunderbolt - Black Clouds over Dark Majesty (1996) - Another gem of polish black metal
Pagan Black Metal:
Bergthron - Durch den Nebel der Finsternis (1995) - Combination of primitive, ultra-raw black metal with some folk melodies and clean vocals
Falkenbach - Laeknishendr (1995) - Falkenbachs raw and weird beginning, more close to black metal than later works. Falkenbachs & Vargas' lost demos are a mysterium itself.
Contemporary black metal (After 2000)
Blutdersommer - Galgensang im Sternenfeuer (2002) - german apocalyptic raw black metal. No album followed after this.
Kallathon - Before drifting into the Abyss (2009) - raw and cold US black metal in the unique style of the Crepuscúlo Negro bands
Kuxan Suum - Kinich ahau (2010) - another, more progressive example of the Crepuscúlo Negro scene.
Obscure stuff:
Grausamkeit - Stardust (2000) - Black Metal meets Video Game music