There are some free autoresponders out there, its been a long time sense I have used one. FreeAutoBot I know people recommend. If you wish to spend a little bit of money I know PHPAutoResponder is a pretty good system for about $50.

If you want a really good autoresponder system I would highly recommend ListMailPro, its a bit price at $195, but its by far the best I have used and you can install it on as many sites as you want and you never have to pay again.