Below is a list of some of the best discussion mailing list hosting options available today...
The standard that others compare against. Incudes many features such as polls, photos, calendar and more.
Simple but useful features like comment voting and custom information pages.
Hosted by a not-for-profit. Uses Sympa open source software with decent web archives.
jigluhood.com (formally mailspaces.com)
A new hybrid wiki and mailing lists with tagging.
Uses open source software modeled after Yahoo groups. Note there limits on the number of subscribers.
Suggest others in the comments below.
Hosting for Online Discussion Groups
If the high majority of the members of your organization or community use the Internet, you may find online discussion groups helpful for hosting communication in between meetings.