Most e-mail clients support mailing lists, which enables you to broadcast e-mail messages to groups that you define. In addition, there are mailing list servers that manage centralized mailing lists for groups of users.

Also see Why E-Mails Bounce in the Did You Know section of Webopedia.

Catalists, the Official Catalog of Listserv lists
Allows you to browse any of the 16,833 public LISTSERV lists on the Internet, search for mailing lists of interest, and get information about LISTSERV host sites. The site is run by L-Soft, makers of LISTSERV software.

listTool.com
listTool.com is a free tool that automates the process of subscribing, unsubscribing and sending commands to over 600 mailing and discussion lists. The site also hosts information about mailing lists, including a good collection of resources and links.

Maintaining a mailing list
This Webmonkey Geek Talk column, from December 12, 1996, provides information on maintaining mailing lists, mail servers and links to popular mailing list maintenance software.

Yahoo!'s mailing lists page
Yahoo!'s directory of mailing lists.