Encyclopaedia Metallum: The Metal Archives
The most complete heavy metal database ever, bar none. No matter how obscure the band or album is, you will most likely find at least a mention here. Their discussion forum is extremely active, too.

The Thrash Metal Guide
The most comprehensive listing of thrash metal bands and albums out there at the moment. The amount and density of information has no comparison, featuring commentary about thousands of bands.

Thrash Unlimited
Dedicated thrash metal forums with a cozy feel, intended for fans to discuss and promote both old and new thrash metal bands. There is also a related internet radio station with very appropriate playlists.

Thrash Never Dies
Latest news from the world of underground thrash metal, among other features. This is largely a community driven project in constant development, and it is starting to show some serious promise.

Dark Lyrics
Unarguably the largest archive of metal lyrics on the web, regardless of the genre. The layout is stylish and simple with no unnecessary fluff or quirks, and finding your way around couldn't be any easier.

Metaleros
The ultimate site about metal bands from Latin America, many of them in the thrash metal category. The coverage of bands and detail of information is awesome and shows an immense amount of dedication.

Metalpage
The serious collector's corner. Plenty of information about the most obscure metal albums with a neat vintage look, and an extensive collectors ring for those who still like to find their stuff the old school way.