I highly recommend
http://www.graphicmail.com It is comparable to Constant Contact but less expensive. To use it, you will just insert code for a sign up box and then go to
http://www.graphicmail.com and choose a template (or create your own) and insert your newsletter info.
I could go on for days about how great
http://www.graphicmail.com is...but I'll spare you

You can also use
http://www.Feedburner.com to create a summary of your recent posts via RSS. It will provide you with code for sign up, but you dont ever have to create a newsletter (feedburner pulls your content and creates a summary version for your newsletter)