Hello,
I am looking for suggestions on a web based newsletter service (eg Constant Contact) that is low cost or free and allows multiple users and multiple lists.

Thank you very much.

check http://www.mailchimp.com/pricing/ they offer Basic plan for free, They have good features..

One of my client use http://www.interspire.com/emailmarketer/ very good support and features.

1-2-all from activecampaign.com they can do hosted or you can host it, or choose from once of their many resellers that do hosted email campaigns

I would agree with the mail chimp solution. When you're looking to try out a new solutions it's always good to have something free to tryout. If you like them then you always have the option to purchase after evaluating what they have to offer.