Archivum.info: Large archive of mailing lists and newsgroups.
CataList: The official catalog of LISTSERV lists.
DiscussionLists.com: Email communities grouped by general topic. Submission form available to suggest new listings.
Dolist.net: References all mailing list types, discussion, diffusion, newsletter, e-zine, alert bulletin, and newsgroups.
Email Newsletters & Ezines at Tile.net: Large mailing list directory.
I Want Newsletters: A large collection of free newsletters sorted by category.
listTool: A directory of lists that includes an easy way of subscribing and unsubscribing to them.
openSubscriber: A mail-to-web gateway with searchable archives of public mailing lists.
Public Mailing Lists Managed by Arrow: A frequently-updated list of publicly accessible mailing lists managed by users of Roger Smith Software's Arrow Mailing List Server.