How about AUTORESPONDER? Have you guys ever been thinking about it?
I realize that ARP is quite a big job, but would be very useful! Especially Newsletter and Autoresponder in one soft.
For now I have a smaller suggestion. Plugin for Qick Cart. First idea would be to send couple of automatic e-mails to your clients. First, informing that bought product has been sent, and another, after e.g. a week, inquiring whether he/she is satisfied about received product. Above is just a simple way to use it. That would make your relations with clients closer. They would feel appreciated etc :). Marketing rules :) What do you think?