As for Heathen, who have already been mentioned, you can download the songs of their first two albums from their official website, just go to
www.heatheninc.com/main/multimedia/mp3.com
I know I´m repeating myself, but further albums (mostly speed/thrash metal) I would recommend are:
Coroner (one of the best thrash metal bands ever!) - anything, but especially the first three albums "R.I.P.", "Punishment For Decadence" and "No More Color"
Anacrusis - anything (you can download all their albums from their official website:
http://apogee.whack.org/~ken/index.html)
Hexenhaus - "A Tribute To Insanity"
Forbidden - "Forbidden Evil"
Death Angel - "The Ultraviolence"
Détente - "Recognize No Authority"
Flotsam & Jetsam - "Doomsday For The Deceiver", "No Place For Disgrace"
Mekong Delta - "Mekong Delta", "The Music Of Erich Zann", "Dances Of Death"
Holy Moses - "Finished With The Dogs"
Voivod - "Killing Technology", "Dimension Hatröss"
Agent Steel - "Skeptics Apocalypse"
Helloween - 1st Ep, "Walls Of Jericho"
Mordred - "Fool´s Game"
Helstar - "Remnants Of War", "A Distant Thunder", "Nosferatu"
Hades - "Restisting Success"
Megadeth - first four albums
Znowhite - "Act Of God"
Testament - "The Legacy" (already mentioned before)
Exodus - "Bonded By Blood" (already mentioned before)
Original Sin - "Sin Will Find You Out"
For some of the most haunting and intense vocals ever, get

"Within The Veil"

by Fear Of God!
It might be difficult to find, but there are some mp3-files on the official website, just go to
www.fear-of-god.com and then click on "Music".
If you want something heavy and slow, I recommend:
Candlemass - "Epicus Doomicus Metallicus", "Nightfall"
Saint Vitus - "Born Too Late", "Mournful Cries", "V"
Solitude Aeturnus - "Into The Depths Of Sorrow", "Beyond The Crimson Horizon", "Through The Darkest Hour"
Zed Yago - "From Over Yonder" (maybe not as "doomy" as the albums mentioned before, but still worth a try; it seems they´re back again -
www.zed-yago.com -, though with a new vocalist - don´t know what they sound like now)