2
I'm using PHPlist
http://tincan.co.uk/phplist at the moment and I think it works really well. lots of configuration options if you like but you can just ignore most of them if you want and go with the basic options.
One problem I'm having with it at the moment is that I've got a large mailing list (almost 11,000 subscribers) and I'm currently having to mess around with the config file to set it to send out mails in batches and / or throttle requests to the server because it keeps exceeding the 'max questions' that my host will allow. This is a common problem with any php mailing list as far as I can tell but there's more and more talk about solutions on their support forum so I guess it's only a matter of time before you can configure throttling and batching through the admin section.
I'd love to see this ported to XOOPS but I'm a total novice with PHP so I can only hope for some other nice techie in here to grant my wish! The only reason I can mess with the config file in PHPlist is because there are examples of hacks on their support forums...