I would like to integrate PHPList into my WP site, but after much searching in PHPList's foruns it looks like a difficult feat for an amateur. However, someone has already written a plugin that does the integration:
However the developer stopped at WP 1.5 (the plugin doesn't work with 1.5.2 or later) and PHPList 2.9.4 (it's now at 2.10.2, with security updates in between).
PHPList is quite a nice newsletter manager, lacking only easy integration, it seems. It would be a great addition to WP to be able to integrate it!