I'm creating an Open Source project, so I don't want to spend too much money, but I've read that a newsletter is one of the most important marketing aspects :-)
Kind regards Johannes
depends what you want to do, but I've used https://sendy.co in the past which uses amazon SES, so minus the upfront cost it's one of the cheapest, as 10,000 mails will cost just $1. However it doesn't have any kind of template designer (or at least it didn't the last time I used it) so if you need that you might have to use something else like campaign monitor or mailchimp, it does support raw html though so you could just copy a template from somewhere, paste it in and add the template tags.
I use mailchimp.
I used MailChimp, GetRevue and Campaign Monitor. There's no perfect newsletter software, though. Every one of them has pros and cons. You should check out a few of them, to find a one what will fit your needs.