I recommend (and I know a lot of other members of this board have done in the past) PowerList which is free from http://www.solutionscripts.com . Although it doesn't have some of the advance features of some other scripts it can hold a very large number of subscibtions.
I use, and have used, a number of MLMs, both Pearl and PHP. The one I like best is SubscribeMe at http://cgi.elitehost.com/. They have a Lite version that I think is free and will probably do what you want it to.
"Whatever can be said can be said clearly." -- Ludwig Wittgenstein