Wikipedia: Black metal is a sub-genre of heavy metal, which properly started in the early 1990's, though its inspirations were formed in the early 1980's.
There are two views on the genre. One views black metal as a very specific form of music that must adhere to a particular 'style' in order for it to be a part of the genre; while the second considers the lyrical and philosophical/political ideology of the music to take more precedence in defining the genre itself, rather than 'style,'
Me again: Basically it's more or less extreme metal with occult or satanistic lyrics, often with pagan themes woven in. Norwegian bands are amongst the most popular, and the viking heritage supposedly is connected to that. In my opinion the norwegians are biggest for two reasons:
1. They make good gengre music and don't care too much about commercialism, which make them appeal to the coll crowd.
2. They were the first to discover that burning down churches were a good way to get notoriety and fame. And this is not a joke! In the earlly 90's there was a string of churches lit on fire in Norway of black metal musicians and followers.
I'm afraid I don't know of any free, legal music, though.