Dada Mail also gets my vote however it's a perl script, not a PHP script. But Dada Mail has really blossomed... especially in the last year. Grab the new version and give it a whirl.
The other script I use is phpList from http://tincan.co.uk/phplist -- in fact, I have transferred my own lists all over to phpList because it offers more advanced list/message management and personalization. I really like phpList. It is quite a bit more complex set-up wise than Dada Mail, but it is a point-and-click installed script offered through Fantastico which is just outstanding.
The only thing about phpList is that it doesn't do batching (send 50 msgs/minute or whatever). However if you search in their forums for "batch" a user has contributed a hack that you can hard-code into the source code which works like a champ.
Both Dada Mail and phpList are GPL.
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