Basic Mailing List software ?
 

I need something easy to use for somebody I'm building a website for ... something basic, where she can send html... and add a little box on her website where they can opt in.

dada mail

Funny, I just ran accross this today, kind of randomly. Looks really nice:

http://www.iceburg.net/pommo/index.php/Main_Page

Dada still. There's another third party one that a lot of people are using, I forget the name.

Dark Mailer.......

dada for sure