There a number of directories of discussion lists. Don’t expect one source to be comprehensive (or even very informative!). General search tools such as Lycos/Infoseek are also useful for locating lists and archives, especially if you know the name of the list you are seeking. Here are a few examples:
(a) Diane Kovacs Directory of Scholarly Electronic Conferences : directory with abstracts of some of the more academic lists. Available on several hosts including: WWW: http://n2h2.com/KOVACS
(b) Tile Listserv directory: Searchable by keyword (remember to load the graphics in order to see the search option). No abstract, but details do give a descriptive title, number of subscribers, list organiser. Also presents lists arranged by a variety of criteria. WWW: http://www.tile.net/tile/listserv/index.html.
(c) Liszt Mailing list directory: of some 72,000 lists. Keyword searchable, plus a broad subject categorisation. Some entries have abstract, most are title/contact details only. WWW: http://www.lizst.com
(d) E-mail list of lists, addresses only: send message to: listserv@uicvm.uic.edu with text list global.
(e) JISCmail lists: (Formerly Mailbase) service provides a keyword search and archives of all the lists (messages, members, help files, organisers). WWW: http://www.jiscmail.ac.uk/.
(f) Usenet Newsgroups: groups are archived on various servers. The Deja News WWW offers a keyword searchable archive (can also search for the name of the newsgroup in which your search terms appear most frequently). WWW: http://www.dejanews.com. [A comprehensive list (with little descriptive information is in the FTP archive at rftm.mit.edu. You can get a search on all usenet lists for messages on a particular subject: send email to netnews@db.stanford.edu with the command “help” in the body of the message. New lists are themselves advertised on the usenet group called news.groups.]
(g) Reference.Com: searchable directory and archive of some 150,000 newsgroups, mailing lists and web forums (seems to be a 12 month archive?). WWW: http://www.reference.com
(h) Publicly Accessible Mailing Lists: http://www.neosoft.com/internet/paml/