External resources that have proven themselves useful during the years, in the order of appearance:
Metal Rules (1995) - a pioneer of the online metal magazine world, definitely worth frequent visits still.
The BNR Metal Pages (1995) - one of the oldest sites dedicated to heavy metal, and it's still going strong.
Thrash or Die! (1997) - a good and thorough site on thrash metal though unfortunately mostly in Japanese.
Metalpage (1999) - the serious collector's corner featuring reviews of vintage albums, a collectors ring etc.
Metal Temple (2000) - a nice and cozy metal webzine featuring news, reviews, interviews... (you name it)
No Life 'til Metal (2000) - possibly the largest personal metal review archive out there, of various genres.
The Metal Observer (2000) - another fine metal webzine notable for its nice reviews, among other things.
Voices From The Darkside (2000) - a stylish webzine with some great interviews of old thrash metal bands.
Classic Albums (2001) - an awesome collection of most rare out-of-print metal albums in RealAudio format.
Dark Lyrics (2001) - a fine and focused metal lyrics archive with no unnecessary fluff or quirks whatsoever.
Metaleros (2001) - an excellent site about metal from Latin America, with a lot of unique background info.
Encyclopaedia Metallum: The Metal Archives (2002) - the ultimate heavy metal database today, bar none.
Ruuth's Inn (2003) - one of the oldest thrash metal webzines, up and running again after a long absence.
The Corroseum (2003) - great info on many obscure metal albums and related topics like old record labels.
Thrash Unlimited (2005) - dedicated thrash metal forums, for fans of both classic and new stuff equally.
Cult Metal (2006) - a site focused on cult metal albums, particularly interesting for the detailed photos.
Metal Treasures (2007) - another noteworthy resource for collectors of rare underground (CD) releases.
The Thrash Metal Guide (2007) - the name says it all; a massive collection of data can be found there.