I definitely agree with a lot the posts already mentioned so I'll try not to repeat too much.
Some I'd like to add that I can think of at the moment are:
Belenos - Errances Oniriques
Borknagar - Quintessence
Darkened Nocturn Slaughtercult - Nocturnal March (this is 2004, but I think it still applies)
Destruction - Metal Discharge
Goatwhore- The Eclipse of Ages into Black
Gojira - I'm really tempted to put "From Mars to Sirius" here even though that's breaking the rules but...
Gorguts - Obscura
Iced Earth - Horror Show
Immolation - Close to a World Below
Immortal- (just 'cause I need to state the obvious) Diabolical Fullmoon Mysticism
Crowbar - Sonic Excess in its Purest Form
Earth - Hex: Or Printing in the Intern (I know 2005... I keep breaking the rules :[ but i totally see this album as "standing the test of time")
Endstille - Fruhlingserwachen I mean this one might be a stretch as I don't hear much talk about Endstille to be honest.
Katatonia - Discouraged Ones (I know this is '98 but... yeah.)
Khanate - Khanate (This just might be just a personal favorite of mine, but anyone who's into drone doom always refer to this band and this album. It's brilliant but definitely not for everyone.)
Kylesa - Kylesa
Lykathea Aflame - Elevenfris
Manilla Road - Spiral Castle *Absolute favorite Manilla Road album
The Meads of Aspohdel - The Excommunication of Christ
Melechesh - Sphynx ?? I'm not sure about this one but I definitely think they did something pretty different in this album.
Nokturnal Mortum - NeChrist
Oxbow - An Evil Heat
Root - Madness of the Graves
Rwake - If You Crawl Before You Walk, You Crawl Before You Die
Shining - The Errie Cold
Watain - Casus Luciferi
Windir - 1184
Ulver - Perdition City
Sigur Ros - Odin's Raven Magic <- this isn't metal, but who cares at this point. Really awesome album.