CataList
The official catalog of LISTSERV lists.

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

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.

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