What

OffTopic is a very simple web widget generator for mailing lists.

How

Subscribe (n.b. do not "invite", just "subscribe". OffTopic do not sends confirmation mails) the address offtopic@informaticalibera.info to your mailing list, and in your website include an iframe such as 

<iframe src="http://ot.madbob.org/w/$the_mail_address_of_your_mailing_list"></iframe>

Which

The mailing list actually subscribed to OffTopic are:

Who

This free service is implemented and operated by Roberto Guido. Code is available under the AGPLv3 license. Read here for the full story behind.