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