Demilich - Great death metal band from Finland. Only released one album, but all of their music (including the album, as well as demos and live recordings) can be downloaded for free on their website. Has some of the most deepest, bowel-levitating vocals I've ever heard.
Crionic - Heavy/thrash metal from the Czech Republic. Earlier releases are generally thrash-oriented. As with Demilich, they only released one album, but Crionic is still active (though nowhere near as thrashy as they were in the early '90s). All of their music is free as well, except it is all encoded at 128k. That stuff aside, check out their album "Different" if you're into thrash.
Acid King - Straight-up doom/stoner metal from San Francisco. They have 1 minute samples for all of their songs, and 5 or 6 whole tracks available for download. One of the few doom bands I've heard which has a female vocalist.
Heathen - Good old Bay Area thrash. All of their stuff is free except for their newest compilation album (which only has samples).
I'll post some more when I feel less lazy.