Currently the world's best online philosophy encyclopedia with many articles about central topics written by the world's leading authors. All articles are updated frequently and are supplied with authoritative bibliographies.
A meta-database with links to a number of both serious and not-so-serious philosophy databases, ranging from lists of personal webpages with online articles to humorous and entertaining philosophy websites.
Another good internet encyclopedia with articles written by serious researchers and often with a more detailed list of topics than what is used in Stanford Enc.Phil (see below).