Email newsletter for website?

How can I add a newsletter to my website? Something automatic that sends a message every time I post something new would be ideal but if you only now how I can create a newsletter that I write myself that would be OK. I need to put a box on my website where people can put their email and have that somehow translate into the newsletter subscriber list. My website has a lot of traffic so it would be too complicated to do it all manually.

Thanks in advance.