Satanism is a broad term that refers to a variety of belief systems whose commonality is that they venerate Satan, or similar figures.
Regular Websites
Representing the church founded by Magus Anton Szandor LaVey, this site introduces the church and its policies on affiliation, its theories and practices, history, the text of interviews given by Anton LaVey, and news relating to the Church of Satan. - Church of Satan
http://www.churchofsatan.com/
Authored by a longtime Satanist, this represents an individual discussion of Satanism today, answering questions about what Satanists believe, what they do, and what they generally do not do. Links to other web sites on the subject are also included. - Modern Satanism
http://www.modernsatanism.com/
Featured here is a description, philosophies, and justification of Satanism. Its pages include an introduction to Satanic ritual and magic, and articles that make the argument that human good is dependent on evil, and not the other way around. - Satanism
http://www.dpjs.co.uk/
The essentials of Satanism are explained, including Satanic philosophy and literature, as well as an online message board for discussions between Satanists and others with an interest in the subject. - Satanism 101
http://web.satanism101.com/
A variety of articles on the subjects of Satanism, witchcraft and demonology are featured here, including Satanic spirituality, leaving Christianity and others. An online chat room is included, along with a blog, videos, and other resources. - Theistic Satanism and Magick
http://www.spiritualsatanist.com/
Theistic Satanism: The New Satanisms of the Era of the Internet
The author, Diane Vera, who describes herself as a theistic Satanist, also known as a traditional Satanist, defines the term as being one who believes in and worships Satan as a deity, and offers articles on a variety of topics related to theistic Satanism. - Theistic Satanism: The New Satanisms of the Era of the Internet
http://www.theisticsatanism.com/