Assuming you're generating these message manually and not looking for some sort of additional integration, I've always liked using PHPList. I've also used CCMail, but I'm not sure it's being developed or supported anymore. It's good software - very intuitive and well put-together, but I get a little nervous when there's no recourse if something goes wrong.
PHPMail is very solid. The only thing you have to remember is that, unfortunately, PHPList's branding can interfere with delivery. See, any kind of open source mailing list software is going to be used by spammers as well as for legitimate means. Because all messages are branded the same, regardless of who is installing and using the software, allowing the PHPList branding to remain within the footer of your messages can actually trigger spam filters. I've noticed messages get delivered 2-3x more when this is removed.
It's a simple procedure to remove it - you set a few variables within the config page, and then you have to make a manual edit to one file to remove text branding. It sucks that it has to be done - I'm all for allowing credit to remain on free apps. But it shoots itself in the foot.
So, after you've done this, you have excellent software. Total install and config time, maybe 15-20 minutes.