If your coop has a hosting account, then they may already have something included with their hosting account.
If not I reccomend
PHP List or Mailman as suggested above. Both are open source and provide everything you should need. Mailman can be a little tricky to configure and not easy for a complete beginner. But when it's set up there should be very little maintenance required since people can subscribe and unsubscribe on their own.
PHPList also provides for HTML emails, but I don't think there is any problem just sending text email. It too can take some effort with all the configuration options, but should also not need much work after it's ready to go. I think PHPList is a little friendier to work with for end users.
Another option for those with smaller lists might be
NewsMerge. This is a program that runs on your PC and not a web server and it's also free. The configuration is very simple and it sends text and HTML emails with little consideration on the part of the enduser. The main points to know about NewsMerge are this:
-The program runs on your PC, not a server.
-The program uses a standard POP3 email account to manage subscriptions.
-The program does not have to run all the time, only when you want to check for subscribe/unsubscribe requests or do a mailing.
- You can import CSV files or your Outlook address book.