For scholarly lists, Dianne Kovacs' Directory of Scholarly Electronic Conferences is comprehensive. A good place to start is the combined search. Usenet Groups Newsreaders have commands to show which of the some 6000 newsgroups are available at your site. For a comprehensive list, try List of Active Newsgroups which is posted periodically to the newsgroup news.answers. You can search all of the guides at once, are restrict yourself to the Humanities, Social Sciences, or Sciences.
Guides typically cover all types of Internet resources.
General Netlink is the kitchen sink of indexes. For more, try Resource Lists . Yahoo was started at Stanford and consists of about 35,000 links, primarily to WWW sites. It can be searched by category or by keyword. Other sites are bigger, but Yahoo is almost always available and fast.