A simple invoicing and shipping application, SAAS at a fairly low rate for tiny businesses.
You need to be able to enter customer data and product/service info, generate a nice, printable invoice and envelope and also offer the option to email the invoice. On the ship side, it should print out a packing list and a shipping label. Bonus points if it an print USPS postage paid labels, or FedEx, or UPS.
A very simple app to be sure (so why the hell haven't I done a basic version for myself yet but still keep doing it manually???) but I've seen enough people online looking for this that I'm convinced it's worthwhile. Go build it and make millions with my blessing!