Encyclopaedia Metallum: The Metal Archives
The ultimate heavy metal database though sometimes painfully slow. 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 jam-packed listing of thrash metal bands and albums out there at the moment. The amount and density of data goes beyond comprehension, featuring commentary about literally 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. (offline)

Thrash Never Dies (closed)
Latest news from the world of underground thrash metal, among several 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 sub 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 most comprehensive 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 immense dedication.

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