I use PHPlist,
phplist.com : Homepage : home , an opensource piece of kit that is both free and manages the subscribe/unsubscribe issue very well, including a nice legal thingy called double opt in, whereby a user subscribes and is then sent a confirmation e-mail which they have to click. It requires a little know how to get it up and running on your server/site, but you can have a few lists like I do, and once you have a nice templet e-mailiung is easy. It also does both plain text e-mails and html ones- though be warned the plain text ones come out like a dogs dinner so you need a line at the top like, @If you can't read this e-mail click here to view as a webpage- then you pop up the newsletter as a webpage- link it into an archive and your site got bigger by a page.
On the legal side of spam, opt in and all. The law is slightly different for a business to business list and a business to consumer list.
Businesses can send unsolicitated e-mails to other businesses as long as there is an an unsubscribe function (which is honored.)
Business to consumer, even association to individual, however requires than the user has requested the e-mail, by say clicking a box that says "I would like the latest news on products from Eco Company X", and then has doubled confirmed (The double opt-in) by clicking on a confirmation link, sent after the event by e-mail to the stated e-mail address.
Please note if the box is one that is already ticked then the default tick has to be on the no box.
Sorry to be boring, I use to do this stuff for a living..
Pete