Here are twenty albums I enjoyed from 2008. There were tons of good albums this year, but not very many that I would call brilliant - 2007 had more of those. This made the ordering very difficult, so it is still very approximate (in some cases arbitrary).
I'm certain I'm forgetting some important releases, and also I just like to hear other people's opinions, so you should all comment with your favorites!
1. Menace Ruine
- In Vulva Infernum AND ALSO Cult of Ruins AND ALSO The Die is Cast
* unique drone and ambient black metal. 3 albums in one year; how prolific!
* awesome anarchist black metal
- Assassins: Black Meddle pt. 1
* very good sludgy metal. Not much black though.
4. Toxic Holocaust
- An Overdose of Death
* crossover-style oldschool thrash
* it's Boris.
6. Slug's Revenge
- Revenge is Never a Four Letter Word
* anarchist folk music played at a punk tempo
- Empire of the Obscene
* thrashy death metal that actually makes me enjoy death metal
8. Orange Tulip Conspiracy
- Orange Tulip Conspiracy
* instrumental experimental rock/fusion that's actually quite interesting. Also has the best album cover of the year.
9. Keny Arkana
* more excellent revolutionary French hip-hop. Not quite as passionate as Entre Ciment et Belle Etoile
10. no cops for miles
- A New Diction
* riot ska, short but very good
* very trippy Finnish psy-trance
12. Black Anvil
- Time Insults the Mind
* really good black thrash; not afraid give a Megadeth style solo
13. A Forest of Stars
- The Corpse of Rebirth
* black metal with very nice use of violin
14. Dreams of the Drowned
* interesting addition of hardcore elements to black metal
- Traced in Air
* neat melodic death metal
16. Quest for Blood
- Quest for Blood
* weird japanese folk black metal with a prominent manic, improvised flute
* beautiful, depressive black metal
- Turning Season Within
* gothic doom metal done right.
- Dragging a Dead Deer Up a Hill
* acoustic shoegaze/ambient/drone
- The Dusty Foot Philosopher
* African-Canadian hip-hop
EDIT: I have now listened to Torche
- Meanderthal (sludge, doom metal), and I think it deserves a spot in my list as well.
EDIT EDIT: Ufomammut
, Genghis Tron
, and probably others I'm not remembering also deserve honorable mention.