These are not mass mailing lists used by commercial spammers, but are topic-oriented discussion lists you can subscribe to. Some of them are broadcast only (for example, PC Magazine Alert and ZDNet AnchorDesk) , while others let you send messages to the list.

Background Information
General background information on what mailing lists are and how they work.
Internet Mailing Lists: Guides and Resources
A useful guide from the International Federation of Library Associations and Institutions
America Online Mailing List Directory
A useful guide to mailing lists, along with a directory of 3,200 you can subscribe to.

Mailing List Catalogs
There are thousands of lists out there to join. . .Here are some guides to help you find the best ones.
CataList
The "official" list of listserv discussion lists, from L-Soft, makers of the listserv software.
List of Lists
Maintained by the author of the book Internet Mailing List Navigator.
Liszt, the mailing list directory
This site offers links to an amazing 71,618 mailing lists you can join. (As of 7/1/97)
Mailing Lists on the Net
A downloadable list of over 7,000 mailing lists.
Publicly Accessible Mailing Lists
A searchable list of hundreds of public mailing lists you can subscribe to.
Reference.COM
You can search the full text of hundreds of newsgroups, mailing lists, and web forums here.
tile.net
A comprehensive database of discussion lists you can search or browse.
W3C World Wide Web Mailing Lists
Over a dozen mailing lists on Web-related topics, from HTML to stylesheets.
Yahoo!: Mailing Lists
Tons of mailing lists, arranged by category.

List Management Services and Software
If you wish to start your own e-mail based discussion list, here are some companies that provide list management:
Cuenet Systems
LISTPROC
L-Soft LISTSERV
ListServe.Com