External resources that have proven themselves worthy during the years:
The BNR Metal Pages (1995-)
One of the oldest sites dedicated to heavy metal, and still going strong.
No Life 'til Metal (2000-)
Possibly the largest personal review archive of metal albums from various genres.
Voices From The Darkside (2000-)
This stylish webzine is notable for some great interviews of old thrash metal bands.
Classic Albums (2001-)
Awesome collection of most rare out-of-print metal albums in RealAudio format.
Dark Lyrics (2001-)
The largest metal lyrics archive on the web, with no unnecessary fluff or quirks.
Excellent information about metal from Latin America, with a lot of unique details.
Encyclopaedia Metallum: The Metal Archives (2002-)
The ultimate heavy metal database today. Their discussion forum is very active, too.
The Corroseum (2003-)
Great info on many obscure metal albums and related topics, like old record labels.
The Thrash Metal Guide (2007-)
The name says it all - a massive collection of data can be found here to dig into.
Metal Forces Magazine (2011-)
The famous metal magazine from the 1980s now online. Their archive contains original reviews of many classic albums and demos.