Atheme.org is a discontinued organisation which maintained the Atheme IRC Platform and other related software.
Information about the Atheme IRC Platform is available at atheme.net.
Information about Charybdis is available at jilles' website.
Atheme's IRCv3 development activity has become it's own organisation. Information about IRCv3 is available at ircv3.net. Most recent Atheme activities are now part of the IRCv3 organisation.