TILE.NET/LISTS
One of the first and largest mailing list directories on the web.

listTool
A directory of lists that includes an easy way of subscribing and unsubscribing to them.

DiscussionLists.com
Email communities grouped by general topic. Submission form available to suggest new listings.

AtoZintheMail.com
Provides newsletters as requested.

Publicly Accessible Mailing Lists
A compilation of contact information for many mailing lists, searchable by mailing list name or subject.

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.

WebScout Lists
Email newsletters and discussion lists which enables subscribe and unsubscribe requests via a simple shopping cart.

Dolist.net
References all mailing list types, discussion, diffusion, newsletter, e-zine, alert bulletin, and newsgroups.

I Want Newsletters
A large collection of free newsletters sorted by category.

CataList
The official catalog of LISTSERV lists.