What's the best way to offer a newsletter to my visitors?
How many newsletters did you subscribe to? My answer to this question is: none. Personally I don't like getting newsletters. I get enough e-mails, no need for more. My opinion is that sending out newsletter e-mails is obsolete. On one hand because e-mail providers reduced the amount of e-mails which can be sent from a certain account to prevent spam and on the other hand people change e-mail addresses quite frequently meaning that some newsletters will never get to the recipient. Other recipients may not even read the newsletters.
Personally I prefer using Social Networks to communicate news about my projects. People who are willing to get the newsletter subscribe to your Facebook page, Twitter account or whatever and get notified when you update it. This also means less work for you. Have a look at the chapter about Social networks for further informations.
For people who are not on Social Networks all the news should also be posted on a News page on your website. I always copy the message I post on Facebook to my News page. The advantage of doing so is that the messages are short and concise. The messages I post on my Facebook page are automatically streamed to my Twitter account.