To hold your existing visitors and create the new one as well you need to have your own mailing list, newsletter, discussion list etc. once you get your website well established. E-Zine Publishing 101 will help you a lot in this field before going for a newsletter of your own. Also some other groups are mentioned hereunder.

egroups - It is free e-zine moderated and immoderated list where infinitive numbers of subscribers communicate.

Liszt - A place where you can get advertisers to promote your mailing list.

Onelist - let you run a free mailing list with an advertisement from this site.

ListBot - With some line of advertisement you can mail 10KB messages daily and provides demographic information of the people who signed up for your newsletter, which is a unique feature.

Mail Server - With a little yearly fee or a banner to be attached on your site, it will give you free mailing privileges and with no extra ads.

WebPromote Engage - It is a hosting service that gives free mailing facilities too, but automatically subscribes or unsubscribes and delivers verification messages etc. Also demographic information are available on request.

Eurofreebies Maillist - It provides you set up your own newsletter in the site and mail messages &:) any announcement to the members (who signed up) from a remotely hosted free mailing list.